Cold and Dense Plasma Sheet Caused by Solar Wind Entry: Direct Evidence

atmosphere

Case Report Cold and Dense Plasma Sheet Caused by Solar Wind Entry: Direct Evidence

Yue Yu 1,2, Zuzheng Chen 1,2,* and Fang Chen 1,2

1 School of Space and Environment, Beihang University, Beijing 100089, China; [email protected] (Y.Y.); [email protected] (F.C.) 2 Key Laboratory of Space Environment Monitoring and Information Processing, Ministry of Industry and Information Technology, Beijing 100089, China * Correspondence: [email protected]

Received: 12 June 2020; Accepted: 2 August 2020; Published: 7 August 2020

Abstract: We present a coordinated observation with the Magnetospheric Multiscale (MMS) mission, located in the Earth’s magnetotail plasma sheet, and the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission, located in the solar wind, in order to understand the formation mechanism of the cold and dense plasma sheet (CDPS). MMS detected two CDPSs composed of two ion populations with diﬀerent energies, where the energy of the cold ion population is the same as that of the solar wind measured by ARTEMIS. This feature directly indicates that the CDPSs are caused by the solar wind entry. In addition, He+ was observed 3 3 in the CDPSs. The plasma density in these two CDPSs are ~1.8 cm− and ~10 cm− , respectively, roughly 4–30 times the average value of a plasma sheet. We performed a cross-correlation analysis on the ion density of the CDPS and the solar wind, and we found that it takes 3.7–5.9 h for the solar wind to enter the plasma sheet. Such a coordinated observation conﬁrms the previous speculation based on single-spacecraft measurements.

Keywords: cold and dense plasma sheet; solar wind entry; coordinated MMS/ARTEMIS observation

1. Introduction The Earth’s magnetotail plasma sheet, which plays a crucial role in the solar wind-magnetosphere coupling, is usually characterized by hot (T 4 keV) and tenuous (N 0.3 cm 3) ions [1]. However, i ≈ i ≈ − prolonged northward interplanetary magnetic field (IMF) conditions can lead to the formation of rare 3 cold and dense plasma sheets (CDPSs) [2–12]. In the CDPS, the ion number density (>1 cm− ) is several times the typical value of a plasma sheet, while the ion temperature (~1 keV) is lower than the typical value of a plasma sheet. The CDPS plays crucial roles in the magnetospheric dynamic processes by causing the following consequences. The CDPS probably makes great contributions to the production of storm-time energetic particles [13] and can lead to stronger ring currents during the main phase of storms [9,14]. The CDPS can also lead to the mass loading of the magnetotail. Therefore, it may have effects on substorms [13,15–17] and reduce the reconnection rate [18,19]. In addition, by altering the plasma density gradients near the dipolarization fronts (DFs), the CDPS can affect the DF dynamics [20], such as the density-driven instabilities [21,22] and electric field structure [23] near the DFs. Meanwhile, since cold electrons behind the DFs cannot be efficiently accelerated by the Fermi and Betatron acceleration mechanism [24–31], the electron distribution behind the DFs [32–34] may be influenced. The formation mechanism of the CDPS has been widely studied. The ionosphere and the solar wind were believed to be possible sources of plasma in the CDPS [13,35]. Large plasma temperature anisotropy, caused by the decrease in parallel temperature in a CDPS, indicates the presence of cold and

Atmosphere 2020, 11, 831; doi:10.3390/atmos11080831 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 831 2 of 13

field-aligned ionospheric origin ion populations [36]. However, it has been found that the ionospheric outflow is strong during the southward IMF period [37,38], which is inconsistent with the observation that the CDPS is caused by prolonged northward IMF periods. In addition, there is a lack of cold O+ component in the CDPS [39,40]. Therefore, the ionosphere may not be the primary source of plasma in the CDPS. Terasawa et al. [10] presented that the plasma density and temperature in the CDPS correlate with those in the solar wind during the northward IMF period, indicating that the solar wind is the source of plasma in the CDPS. In addition, cold ions (e.g., H+ and He2+) with a lower solar-wind-like energy were widely observed in the CDPS [4,5,41]. Fujimoto et al. [2] found that two ion populations with distinctly different energies coexist in a CDPS, with these lower energy ions being similar to those in the magnetosheath. However, there has been no simultaneous observation of the plasma energy spectrum in both the solar wind and the CDPS. Therefore, there is no direct comparison between the energy of cold ion populations in the CDPS and the energy of ions in the solar wind. To verify the ion energy of these two populations, two satellites located, respectively, in the plasma sheet and the solar wind should be used. With the launch of the Magnetospheric Multiscale (MMS) mission [42–44], which frequently travelled to the Earth’s magnetotail plasma sheet during the summer (May–September) of 2017, and the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission [45], which travelled around the Moon and was certainly frequently located in the solar wind, we are able to investigate this issue. In this study, based on coordinated MMS/ARTEMIS observations, we present two CDPS events in order to provide direct evidence that the CDPS is a consequence of solar wind entry.

2. Observations For the plasma sheet measurements, we use the MMS mission [42]. Specifically, the magnetic field data with a 0.0625 s resolution are from Fluxgate Magnetometer (FGM) [43], and the plasma data with a 4.4 s resolution are from Fast Plasma Investigation Instrument (FPI) [44]. For the solar wind measurements, we use the ARTEMIS mission [45]. Specifically, magnetic field data with a 4.3 s resolution and plasma data with a 4.3 s resolution are from Fluxgate Magnetometer (FGM) [46] and Electrostatic analyzers (ESA) [47], respectively. All the data are shown in Geocentric Solar Magnetospheric (GSM) coordinates unless noted otherwise. We present two cold and dense plasma sheet (CDPS) events detected by MMS, when ARTEMIS was in the solar wind. Therefore, a direct comparison of the ion energy spectrum between the plasma sheet and the solar wind can be obtained. Specifically, the first event was observed by MMS from 10:10:00UT to 11:50:00 UT on 20 July 2017, when MMS spacecrafts were moving from ( 21.655, 6.838, 4.078) R to ( 22.151, 6.636, 4.273) R − E − E (Earth radii) in Geocentric Solar Magnetospheric (GSM) coordinates (see red pentagram in Figure1), indicating that MMS spacecrafts were located on the dusk-side of the magnetotail. At the same time, ARTEMIS was moving from (42.503, 40.308, 5.009) R to (42.813, 39.733, 5.006) R in GSM − − E − − E coordinates (see yellow pentagram in Figure1), indicating that ARTEMIS was located in the solar wind. Atmosphere 2020, 11, 831 3 of 13 Atmosphere 2020, 11, x FOR PEER REVIEW 3 of 14

Figure 1. The locations of MMS (denoted by red pentagram) and ARTEMIS (denoted by yellow Figure 1. The locations of MMS (denoted by red pentagram) and ARTEMIS (denoted by yellow pentagram) in the GSM coordinates on 20 July 2017. The cyan grid region denotes the magnetosphere. pentagram) in the GSM coordinates on 20 July 2017. The cyan grid region denotes the The location of the magnetopause is calculated with Sibeck’s model [48]. GSM = Geocentric magnetosphere. The location of the magnetopause is calculated with Sibeck’s model [48]. GSM = Solar Magnetospheric. Geocentric Solar Magnetospheric. Figure2 presents an overview of the event, with the magnetic field shown in panel a, the ion and electronFigure 2 numberpresents density an overview in panel of b,the electron event, wi temperatureth the magnetic in panel field c, shown ion temperature in panel a, in the panel ion and electron number density in panel b, electron temperature in panel c, ion temperature in panel d, d, Ti/Te in panel e, ion velocity in panel f, pitch angle distribution of 2–200 eV electrons in panel g,Ti/T ione in di panelfferential e, ion energy velocity flux in (0.01–30panel f, pitch keV) inangle panel distribution h, and plasma of 2–200β in eV panel electrons i. These in panel data g, were ion collecteddifferential by energy MMS1 flux in the (0.01–3 fast mode0keV) [ 42in]. panel During h, and this plasma period, β MMS in panel measured i. These a small data magneticwere collected field by MMS1 in the fast mode [42]. During this period, MMS measured a small magnetic3 field Bx (|Bx|< Bx (|Bx| < 20 nT; see blue line in Figure2a), high plasma densities (N & 1 cm− ; Figure2b), and large ≳ −3 plasma20 nT; seeβ (blueβ > 0.5line; Figurein Figure2i), 2a), indicating high plasma that it densities was in the (N plasma 1 cm sheet; Figure [ 49 2b),]. After and large 11:40:00 plasma UT, twoβ (β DFs> 0.5; (Figure Figure2 a),2i), characterizedindicating that by it awas sharp in increasethe plasma in thesheet Z-component [49]. After 11:40:00 of the magnetic UT, two fieldDFs [(Figure24,50], were2a), characterized detected in a by flow a burstsharp (Figureincrease2f) in [ 49 the]. AtZ-component the beginning of (beforethe magnetic 10:30:00 field UT) [24,50], and the were end detected in a flow burst (Figure 2f) [49]. At the beginning (before 10:30:003 UT) and the end (after (after 11:30:00) of this interval, the plasma number density (1–2.2 cm− ; Figure2b) is higher than the 11:30:00) of this interval, the plasma 3number density (1–2.2 cm−3; Figure 2b) is higher than the average in the plasma sheet (~0.3 cm− [1]), while both the ion (1–1.8 keV; Figure2d) and electron average in the plasma sheet (~0.3 cm−3[1]), while both the ion (1–1.8 keV; Figure 2d) and electron temperature (130–230 eV; Figure2c) are lower than the average in the plasma sheet (T i ~3 keV, Te ~ temperature (130–230 eV; Figure 2c) are lower than the average in the plasma sheet (Ti ~3 keV, Te ~3 500 eV [1]). Between 10:30:00 UT and 11:30:00, the plasma density rapidly increases from 1.4 cm− 500 eV [1]).3 Between 10:30:00 UT and 11:30:00, the plasma density rapidly increases from 1.4 cm−3 to to 1.8 cm− (Figure2b), i.e., 4.5–6 times the average of the plasma sheet. In addition, the ion and −3 electron1.8 cm temperatures(Figure 2b), i.e., decrease 4.5–6 to times ~800 the eV (Figureaverage2d) of and the ~100 plasma eV (Figuresheet. 2Inc), addition, respectively. the Theion ionand spectrumelectron temperatures features two decrease ion populations to ~800 witheV (Figure distinctly 2d) diandfferent ~100 energieseV (Figure (Figure 2c), respectively.2h), whose cold The and ion hotspectrum ion population features two energies ion populations are ~1 keV with and ~3distinct keV,ly respectively. different energies These features(Figure 2h), are whose consistent cold with and thosehot ion in population the CDPS [energies10,35,51– are54], ~1 which keV and is widely ~3 keV, believed respectively. to bethe These consequence features are of consistent the solar wind with entrythose [in12 ,the51, 52CDPS]. During [10,35,51–54], the crossing which of the is widely CDPS (10:10:00–11:50:00 believed to be the UT), consequence the temperature of the solar of the wind cold ionentry populations [12,51,52]. During was ~400 the eV crossing (green of curve the CDPS in Figure (10:10:00–11:50:002d), while that UT), of the the hot temperature ion populations of the wascold ~11,500ion populations eV (blue was curve ~400 in FigureeV (green2d). curve As can in beFigure seen, 2d), the while ratios that of the of ionthe tohot electron ion populations temperature was and~11,500 cold eV ion (blue to electron curve in temperature Figure 2d). As were can ~9 be and seen ~4,, the respectively. ratios of the In ion addition, to electron these temperature electrons with and acold low ion energy to electron (Figure temperature2g) appeared were mainly ~9 and in ~4, the re parallelspectively. and In anti-parallel addition, these directions, electrons showing with a thelow signatureenergy (Figure of a cigar 2g) distributionappeared mainly [24,32– 34in,55 the]. Suchparallel a cigar and distribution anti-parallel of electronsdirections, with showing low energy the wassignature also observed of a cigar in distribution previous CDPSs [24,32–34,55]. caused by Such the solar a cigar wind distribution entry via aof low-latitude electrons with boundary low energy layer (LLBL)was also [56 observed]. in previous CDPSs caused by the solar wind entry via a low-latitude boundary layer (LLBL) [56].

Atmosphere 2020, 11, 831 4 of 13 Atmosphere 2020, 11, x FOR PEER REVIEW 4 of 14

Figure 2. MMS1 observations of a cold and dense plasma sheet (CDPS) on 2020 JulyJuly 2017.2017. (a) Magnetic ﬁeldfield BBxxcomponent component (blue (blue curve), curve), By component(greenBy component(green curve), curve), and B zandcomponent(red Bz component(red curve); (curve);b) electron (b) (blackelectron curve) (black and curve) ion (green and ion curve) (green number curve) density; number (c) density; electron ( temperature;c) electron temperature; (d) ion temperature (d) ion (black),temperature cold ion(black), temperature cold ion (green),temperature and hot (green), ion temperature and hot ion divided temperature by 5 (blue); divided (e) by the 5 ratio (blue); of the(e) ionthe toratio electron of the temperature ion to electron (black) andtemperature the ratio of(bla theck) cold and ion the to electronratio of temperature the cold ion (green); to electron (f) ion

velocitytemperature Vx component (green); (f) ion (blue velocity curve), V Vx componenty component (blue (green curve), curve), Vy component and Vz component (green curve), (red curve);and Vz (componentg) pitch angle (red distribution curve); (g) pitch of electrons angle distribution with an energy of elec oftrons 2–200 with eV; (anh) energy ion diﬀ erentialof 2–200energy eV; (h) ﬂuxion (0.01–30differential keV); energy and (fluxi) plasma (0.01–30β. ThekeV); orbit and of (i) MMS plasma is presented β. The orbit at theof MMS bottom is presented of panel i. at the bottom of panel i. The global MHD model with kinetic test particles [8] showed that the cold ion population originatesThe global from theMHD solar model wind throughwith kinetic LLBL. test Previous particles observations [8] showed found that that the thecold energy ion population of the cold ionoriginates population from inthe the solar CDPS wind is similar through to thatLLBL. of Previo ions inus the observations solar wind [found4,5,41]. that However, the energy there of is the no directcold ion comparison population of in these the ionCDPS populations is similar betweento that of the ions CDPS in the and solar the wind solar wind.[4,5,41]. However, there is noFigure direct 3comparison shows a direct of these comparison ion populations of these between ion populations the CDPS between and the thesolar CDPS wind. and the solar wind,Figure using 3 the shows plasma a direct sheet comparison data collected of bythese MMS1 ion populations in the fast mode between [42] andthe CDPS the solar and wind the solar data collectedwind, using by ARTEMISthe plasma P1 sheet in the data reduced collected mode by MMS1 [45]. As in can the be fast seen, mode the [42] CDPS and was the detectedsolar wind during data northwardcollected by IMF ARTEMIS periods P1 (B inz >the0; reduced Figure3 a),mode consistent [45]. As with can be previous seen, the studies CDPS (Wing was detected et al. [ 12 during] and therein).northward The IMF energy periods of ions (Bz in> the0; Figure solar wind 3a), consistent (Figure3b) with is indeed previous consistent studies with (Wing that ofet coldal. [12] ions and in thetherein). CDPS The (Figure energy3c), of directly ions in demonstrating the solar wind that (Figure these 3b) cold is indeed ions in consistent the CDPS maywith originatethat of cold from ions the in solarthe CDPS wind. (Figure To verify 3c), our directly inference, demonstrating we checked that He +thesein this cold CDPS, ions whichin the comesCDPS frommay theoriginate solar wind.from the solar wind. To verify our inference, we checked He+ in this CDPS, which comes from the solar wind. He+ was observed when the CDPS was detected (Figure 3d), which further proves that these

Atmosphere 2020, 11, 831 5 of 13

He+ was observed when the CDPS was detected (Figure3d), which further proves that these cold ions in the CDPS originate from the solar wind. The distribution of cold ions is intermittent (Figure3c,d), which may attributable to the spatial discontinuity of cold ions. The solar wind may cross the flank of the bow shock and then enter the plasma sheet. It has been suggested that the solar wind plasma can enter the plasma sheet via the low-latitude boundary layer (LLBL) during northward IMF periods [3,57]. Thus, the ion number density in the CDPS should be correlated with that in the solar wind. Based on the correlation, we can estimate the duration for the solar wind ion to propagate from the LLBL to the plasma sheet, which is to say we can search for which period the ion number density in the solar wind has the best correlation with ion number density in the CDPS. We perform a cross-correlation analysis between the ion number density in the CDPS and ion number density in the solar wind. The cross-correlation coefficient is defined as follows:

 Pt Nsw(M t+i)Nps(i)  i=1 −  q P P , t N  [ t N (M t+i)N (M t+i)][ t N (i)N (i)] ≤  i=1 sw sw i=1 ps ps R (t)=  − − (1) sp,coe f f  PN  i= Nsw(M t+i)Nps(i)  q 1 − , N < t M  PN PN  [ Nsw(M t+i)Nsw(M t+i)][ Nps(i)Nps(i)] ≤ i=1 − − i=1

Nsw, Nps, M, N, and t represent the ion number density in solar wind, ion number density in CDPS, time scale of Nsw, time scale of Nps, and time lag. Since the ion data resolution of ARTEMIS (4.3 s resolution) is higher than that of MMS (4.4 s resolution), we perform the linear interpolation to reduce the ARTEMIS resolution. The time-series of the ion number density in the CDPS (10:10:00–11:50:00 UT) slides back along that of the ion number density in the solar wind from the time that the CDPS was detected. During this process, the cross-correlation coefficient first increases and then decreases. The largest cross correlation coefficient (~0.62) occurs at lags 4.1 h (Figure3e), indicating that it takes 4.1 h for ions to propagate ≈ from the ARTEMIS location to the plasma sheet. Considering the fact that the solar wind velocity is stable, (–350 60 25] km/s, we can estimate that the duration for ions to propagate from the ARTEMIS − location to the LLBL is ~24 min. Therefore, it takes 3.7 h for ions to propagate from the LLBL to the plasma sheet. The energy of ions in the solar wind 4.1 h ago (Figure 7e) is still consistent with that of cold ions in the CDPS, suggesting that the results of this cross-correlation analysis are reasonable. The second event was observed by MMS from 02:10:00 to 05:30:00 UT on 4 August 2017, when MMS spacecrafts were moving from ( 22.589, 8.767, 5.821) R to ( 22.328, 7.805, 5.923) R in Geocentric Solar − E − E Magnetospheric (GSM) coordinates (see red pentagram in Figure4), indicating that MMS spacecrafts were located on the duskside of the magnetotail. At the same time, ARTEMIS was moving from ( 48.567, 38.941, 4.390) R to ( 49.941, 38.043, 4.257) R in GSM coordinates (see yellow pentagram in − E − E Figure4), indicating that ARTEMIS was located in the solar wind. Figure5, in the same format as Figure2, presents an overview of the event. These data were collected by MMS1 in the fast mode [ 42]. During this period, MMS measured a small magnetic field Bx (|Bx| < 20 nT; see blue line in Figure5a), 3 high plasma densities (N & 1 cm− ; Figure5b), and a large plasma β (β > 0.5; Figure5i), indicating that it 3 was in the plasma sheet [49]. The plasma density is 3~10 cm− (Figure5b), i.e., 10–30 times the average value of the plasma sheet. In addition, the ion and electron temperatures are 400–800 eV (Figure5d) and 20–70 eV (Figure5c), respectively. The ion spectrum features two ion populations with distinctly different energies (Figure5h), whose cold and hot ion population energies are 0.1–1.2 keV and 6–20 keV, respectively. These features are consistent with those in the CDPS [10,51–53]. During the crossing of the CDPS, the temperature of the cold ion populations was ~200 eV (green curve in Figure2d), while that of the hot ion populations was ~5 keV (blue curve in Figure2d). As can be seen, the ratios of the ion to electron temperature and cold ion to electron temperature were ~15 and ~5, respectively. In addition, these low-energy electrons (Figure2g) showed the signature of a cigar distribution, which was also observed in previous CDPSs caused by solar wind entry via the low-latitude boundary layer. Figure6, Atmosphere 2020, 11, 831 6 of 13 in the same format as Figure4, shows a direct comparison of these ion populations between the CDPS sheet and the solar wind, using the plasma sheet data collected by MMS1 in the fast mode [42] and the solar wind data collected by ARTEMIS P1 in the reduced mode [45]. As can be seen, the CDPS was detected during northward IMF periods (Bz > 0; Figure6a), consistent with previous studies (Wing et al. [12] and therein). The energy of ions in the solar wind (Figure6b) is indeed consistent with that of cold ions in the CDPS (Figure6c), directly demonstrating that cold ions in the CDPS originate from solar wind. In addition, He+, mainly from the solar wind, was observed when the CDPS was detected (Figure6d), which also demonstrates that these cold ions in the CDPS originate from the solar wind. We also performed a cross-correlation analysis on the ion number density between the CDPS and the solar wind. We selected the time-series of the CDPS from 02:10:00 to 05:30:00 UT, sliding it back along that of the ion number density in the solar wind from the time that the CDPS was detected. As can be seen, ARTEMIS is located downstream of MMS, indicating that the ions observed by ARTEMIS cannot propagate toward MMS. However, these upstream cold ions near the LLBL can propagate to the downstream ARTEMIS and can simultaneously leak into the plasma sheet. The time needed for these upstream cold ions near the LLBL to propagate to ARTEMIS is defined as Ta, while the time needed for these upstream cold ions near the LLBL to propagate to the MMS is defined as Tm. Thus, the time lag obtained by the cross-correlation analysis is Tm-Ta. Then, we can obtain the time needed for the upstream cold ions near the LLBL to propagate to MMS by adding Ta to Tm-Ta. The largest cross correlation coefficient (~0.8) occurs at lags 5.7 h (Figure6e), namely Tm-Ta = 5.7 h. Considering the ≈ fact that the solar wind velocity is stable, ( 360 60 0) km/s, we can estimate that the duration needed − for ions to propagate from the LLBL to the ARTEMIS location is ~14 min (Ta). Therefore, it takes 5.9 h for ions to propagate from the LLBL to the plasma sheet. The energy of ions in the solar wind 5.7 h ago (Figure7j) is still consistent with that of cold ions in the CDPS, suggesting that the results of this cross-correlationAtmosphere 2020, 11, x analysis FOR PEER are REVIEW reasonable. 6 of 14

Figure 3. Direct comparison of the ion populations betw betweeneen the CDPS and the solar wind on 20 July 2017. ARTEMISARTEMIS observations observations of (ofa) the(a) Interplanetarythe Interplanetary Field (IMF)Field B(IMF)z component Bz component and (b) ion and diff erential(b) ion energydifferential flux energy in the solarflux in wind. the solar MMS1 wind. observations MMS1 observations of (c) the ion of di(c)ff theerential ion differential energy flux energy in the flux CDPS in andthe CDPS (d) He and+ number (d) He density.+ number (e )density. Cross-correlation (e) Cross-correlation coefficient of coefficient the ion number of the densityion number between density the CDPSbetween and the the CDPS solar and wind. the The solar orbit wind. of ARTEMISThe orbit of is ARTEMIS presented atis presented the bottom at of the panel bottom c. of panel c.

The second event was observed by MMS from 02:10:00 to 05:30:00 UT on 4 August 2017, when MMS spacecrafts were moving from (−22.589, 8.767, 5.821) RE to (−22.328, 7.805, 5.923) RE in Geocentric Solar Magnetospheric (GSM) coordinates (see red pentagram in Figure 4), indicating that MMS spacecrafts were located on the duskside of the magnetotail. At the same time, ARTEMIS was moving from (−48.567, 38.941, 4.390) RE to (−49.941, 38.043, 4.257) RE in GSM coordinates (see yellow pentagram in Figure 4), indicating that ARTEMIS was located in the solar wind. Figure 5, in the same format as Figure 2, presents an overview of the event. These data were collected by MMS1 in the fast mode [42]. During this period, MMS measured a small magnetic field Bx (|Bx| < 20 nT; see blue line in Figure 5a), high plasma densities (N≳1 cm−3; Figure 5b), and a large plasma β (β > 0.5; Figure 5i), indicating that it was in the plasma sheet [49]. The plasma density is 3~10 cm−3 (Figure 5b), i.e., 10–30 times the average value of the plasma sheet. In addition, the ion and electron temperatures are 400–800 eV (Figure 5d) and 20–70 eV (Figure 5c), respectively. The ion spectrum features two ion populations with distinctly different energies (Figure 5h), whose cold and hot ion population energies are 0.1–1.2 keV and 6–20keV, respectively. These features are consistent with those in the CDPS [10,51–53]. During the crossing of the CDPS, the temperature of the cold ion populations was ~200 eV (green curve in Figure 2d), while that of the hot ion populations was ~5 keV (blue curve in Figure 2d). As can be seen, the ratios of the ion to electron temperature and cold ion to electron temperature were ~15 and ~5, respectively. In addition, these low-energy electrons (Figure 2g) showed the signature of a cigar distribution, which was also observed in previous CDPSs caused by solar wind entry via the low-latitude boundary layer. Figure 6, in the same format as Figure 4, shows a direct comparison of these ion populations between the CDPS sheet and the solar wind, using the plasma sheet data collected by MMS1 in the fast mode [42] and the solar wind data collected by ARTEMIS P1 in the reduced mode [45]. As can be seen, the CDPS was detected during northward IMF periods (Bz > 0; Figure 6a), consistent with previous studies (Wing et al. [12] and therein). The energy of ions in the solar wind (Figure 6b) is indeed consistent with that of cold ions in the CDPS (Figure 6c), directly demonstrating that cold ions in the CDPS originate from solar wind. In addition, He+, mainly from the solar wind, was observed when the CDPS was detected (Figure 6d), which also demonstrates that these cold ions in the CDPS originate from the solar wind. We also performed a

Atmosphere 2020, 11, x FOR PEER REVIEW 7 of 14 cross-correlation analysis on the ion number density between the CDPS and the solar wind. We selected the time-series of the CDPS from 02:10:00 to 05:30:00 UT, sliding it back along that of the ion number density in the solar wind from the time that the CDPS was detected. As can be seen, ARTEMIS is located downstream of MMS, indicating that the ions observed by ARTEMIS cannot propagate toward MMS. However, these upstream cold ions near the LLBL can propagate to the downstream ARTEMIS and can simultaneously leak into the plasma sheet. The time needed for these upstream cold ions near the LLBL to propagate to ARTEMIS is defined as Ta, while the time needed for these upstream cold ions near the LLBL to propagate to the MMS is defined as Tm. Thus, the time lag obtained by the cross-correlation analysis is Tm-Ta. Then, we can obtain the time needed for the upstream cold ions near the LLBL to propagate to MMS by adding Ta to Tm-Ta. The largest cross correlation coefficient (~0.8) occurs at lags ≈ 5.7 h (Figure 6e), namely Tm-Ta = 5.7 h. Considering the fact that the solar wind velocity is stable, (−360 60 0) km/s, we can estimate that the duration needed for ions to propagate from the LLBL to the ARTEMIS location is ~14 min (Ta). Therefore, it takes 5.9 h for ions to propagate from the LLBL to the plasma sheet. The energy of ions Atmospherein the solar2020 wind, 11, 831 5.7 h ago (Figure 7j) is still consistent with that of cold ions in the CDPS, suggesting7 of 13 that the results of this cross-correlation analysis are reasonable.

FigureFigure 4.4. The locations of MMS (denoted by redred pentagram)pentagram) andand ARTEMISARTEMIS (denoted(denoted byby yellowyellow Atmospherepentagram)pentagram) 2020, 11, in inx FOR thetheGSM GSMPEERcoordinates REVIEWcoordinates onon 44 AugustAugust 2017.The2017.Theﬁgure figure is is in in the the same same format format as as Figure Figure1 .1. 8 of 14

Figure 5. MMS1 observations ofof aa CDPSCDPS onon 44 AugustAugust 2017.The 2017.The ﬁgure figure is is in in the the same same format format as as Figure Figure2. 2.

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FigureFigure 6.6.Direct Direct comparison comparison of of the the ion ion populations populations between between the CDPS the CDPS and the and solar the wind solar on wind 4 August on 4 2017.August The 2017. ﬁgure The is figure in the sameis in th formate same as format Figure as3. Figure 3.

3.3. SummarySummary andand DiscussionDiscussion InIn thisthis study, study, we we analyze analyze two two cold cold and and dense dense plasma plasma sheets sheets (CDPSs) (CDPSs) via an MMSvia an mission MMS mission during northwardduring northward IMF periods, IMF periods, and we and find we that find the that energy the energy of cold of cold ion populationsion populations inthese in these CDPSs CDPSs is consistentis consistent with with that that of ion of populationsion populations observed observed by the by ARTEMIS the ARTEMIS mission mission in the solar in the wind, solar directly wind, + demonstratingdirectly demonstrating that the CDPSthat the is CDPS the consequence is the consequence of the solar of the wind solar entry. wind In entry. addition, In addition, He , mainly He+, frommainly the from solar the wind, solar was wind, observed was observed in these in CDPSs. these CDPSs. In these In CDPSs, these CDPSs, the plasma the plasma density density was ~4–30 was times~4–30 thetimes average the average plasma plasma sheet value. sheet Wevalue. performed We performed a cross-correlation a cross-correlation analysis analysis of the ion of densitythe ion betweendensity between the CDPS the andCDPS the and solar the wind,solar wind, and foundand foun thatd that it takes it takes 3.7–5.9 3.7–5.9 h forh for solar solar wind wind ions ions toto enterenter thethe plasmaplasma sheet.sheet. SuchSuch coordinatedcoordinated observationsobservations confirmconfirm thethe previousprevious speculationspeculation basedbased onon single-spacecraftsingle-spacecraft measurements.measurements. TheThe pathpath of of solar solar wind wind ions ions entering entering the the plasma plasma sheet sheet depends depends on theon IMFthe IMF conditions. conditions. On theOn one the hand,one hand, during during southward southward IMF periods, IMF solarperiods, wind solar ions enterwind the ions magnetosphere enter the magnetosphere via magnetopause via magneticmagnetopause reconnection magnetic [58 reconnection] and are then [58] convected and are then to the convected magnetotail. to the Subsequently, magnetotail. theseSubsequently, ions are propagatedthese ions are towards propagated the tail towards center andthe tail then center energized and then and energized thermalized and in thermalized the plasma in sheet the [plasma59,60]. Onsheet the [59,60]. other hand, On the during other northward hand, during IMF periods,northward there IMF are twoperiods, major there entry are mechanisms: two major (1) entry the doublemechanisms: cusp reconnection,(1) the double which cusp reconnection, captures magnetosheath which captures plasma, magnetosheath and (2) the plasma, Kelvin–Helmholtz and (2) the instability,Kelvin–Helmholtz which makes instability, solar wind which ions makes “leak” solar into wi thend plasma ions “leak” sheet viainto the the low-latitude plasma sheet boundary via the layerlow-latitude (LLBL) alongboundary the flanks layer (LLBL) of the magnetosphere along the flanks [42 of,57 the]. magnetosphere [42,57]. TheThe globalglobal MHDMHD modelmodel withwith kinetickinetic testtest particlesparticles [[8]8] showedshowed thatthat Cusp-enteringCusp-entering ionsions areare predominantlypredominantly deflecteddeflected towardtoward thethe dawn-sidedawn-side andand areare heated,heated, whilewhile Flank-enteringFlank-entering ionsions viavia LLBLLLBL areare dawn-duskdawn-dusk symmetricsymmetric andand relativelyrelatively cold.cold. TheseThese mechanismsmechanisms leadlead toto ionion populationspopulations inin thethe plasmaplasma sheetsheet exhibitingexhibiting aa dawn-duskdawn-dusk asymmetryasymmetry [[88,61,61,62]:,62]: thesethese ionion populationspopulations atat thethe dusk-sidedusk-side exhibitexhibit aa two-component two-component structure structure of of energy energy distribution distribution due due to theto the cold cold Flank-entering Flank-entering ions ions and and the higher-energythe higher-energy subset subset of Cusp-entering of Cusp-entering ions that ions drift fromthat thedrift dawn-side, from the whiledawn-side, the ion populationswhile the ion at thepopulations dawn-side at exhibit the dawn-side a hotter and exhibit broadly a hotter peaked and energy broadly distribution peaked energy due to thedistribution cold Flank-entering due to the ionscold andFlank-entering the lower-energy ions and subset the lower-energy of Cusp-entering subset ions. of Cusp-entering In these two events, ions. In the these cold two dense events, plasma the sheetscold dense were plasma detected sheets at the were dusk-side detected of theat the plasma dusk-side sheet duringof the plasma northward sheet IMF during periods northward (Figures IMF3a andperiods6a). Thus,(Figures the 3a cold and component 6a). Thus, more the likelycold comp constitutesonent themore flank-entering likely constitutes ions via the KHI, flank-entering while the hotions component via KHI, while mayconstitute the hot component the Cusp-entering may constitute ions that the drift Cusp-entering from the dawn-side. ions that drift from the dawn-side.

Atmosphere 2020, 11, 831 9 of 13 Atmosphere 2020, 11, x FOR PEER REVIEW 10 of 14

Li etet al.al. [35[35]] performed performed MHD MHD simulations simulations on on the th entrye entry rate rate of the of double the double cusp reconnectioncusp reconnection under underdifferent different solar wind solar conditions, wind conditions, showing showing that the entrythat the rate entry peaked rate under peaked the followingunder theconditions: following conditions:(1) the IMF (1) magnitude the IMF wasmagnitude 10 nT, (2)was the 10 IMF nT, clock(2) the angle IMF wasclock 0 ◦angle, (3) the was ion 0°, number (3) the densityion number was 3 density6.6 cm− was, (4) the6.6 solarcm−3, wind(4) the speed solar was wind 500 speed km/s, andwas (5) 500 the km/s, dipole and tilt (5) angle the wasdipole 15◦ .tilt For angle the first was event, 15°. Forwe checkedthe first theevent, solar we wind checked condition the solar when wind the co IMFndition was northwardwhen the IMF ~4 h was ago: northward (1) the IMF ~4 magnitude h ago: (1) thewas IMF ~4 nT magnitude (Figure7a), was (2) ~4 the nT IMF (Figure clock angle7a), (2) varied the IMF from clock ~80 ◦angleto ~0 ◦varied(Figure from7b), ~80° (3) the to ion~0° number (Figure 3 7b),density (3) the was ion ~3 cmnumber− , (4) density the solar was wind ~3 speedcm−3, was(4) the ~358 solar km /winds (not speed shown), was and ~358 (5) thekm/s dipole (not tiltshown), angle andwas (5) ~20.36 the ◦dipole(not shown). tilt angle Such was conditions ~20.36° (not deviate show largelyn). Such from conditions the best deviate conditions largely presented from the in Libest et conditionsal. [35], indicating presented that in the Li entryet al. rate[35], ofindicating the double that cusp the reconnectionentry rate of wasthe double low. For cusp the secondreconnection event, waswe checked low. For the the solar second wind event, conditions we checked when thethe IMFsolar was wind northward conditions ~5.7 when h ago: the (1) IMF the was IMF northward magnitude ~5.7was ~30h ago: nT (1) (Figure the IMF7f), (2)magnitude the IMF clockwas ~30 angle nT was(Figure ~55 7f),◦ (Figure (2) the7g), IMF (3) clock the ion angle number was density~55° (Figure was 3 ~157g), cm(3)− the, (4) ion the number solar wind density speed was was ~15 ~365 cm km−3, (4)/s (not the shown),solar wind and speed (5) the was dipole ~365 tilt km/s angle (not was shown), ~10.02◦ and(not (5) shown). the dipole Such tilt conditions angle was also ~10.02° deviate (not largely shown). from Such the bestconditions conditions also presenteddeviate largely in Li etfrom al. [35the], bestindicating conditions that the presented entry rate in of Li the et double al. [35], cusp indi reconnectioncating that wasthe low.entry Thus, rate theof cuspthe double reconnection cusp reconnectionmay contribute was little low. to Thus, CDPSs. the However, cusp reconnection the simulation may ofcontribute Li et al. [ 35little] is basedto CDPSs. on the However, long-lasting the simulationnorthward of IMF. Li et Such al. [35] a condition is based is on di thefferent long-lasting from that northward in our events, IMF. whichSuch a were condition observed is different shortly fromafter IMFthat turnedin our from events, southward which towere northward. observed Therefore, shortly weafter can IMF only turned obtain afrom rough southward estimation to of northward.the entry rate. Therefore, we can only obtain a rough estimation of the entry rate.

Figure 7. ParametersParameters of of the the solar wind observedobserved by ARTEMIS. On 20 July 2017: ( a)Magnetic field ﬁeld

strength (blue) and magnetic field field Bz componentcomponent (red); (red); ( b)) Interplanetary magnetic magnetic field field (IMF) clock angle; ((cc)) ionion number number density; density; (d )(d the) the ratio ratio of the of ionthe toion electron to electron temperature; temperature; (e) ion (e di) ffionerential differential energy

energyflux (0.01–30 flux (0.01–30 keV).On keV).On 3 August 3 2017:August (f) Magnetic2017: (f) Magnetic field strength field (blue) strength and (blue) magnetic and field magnetic Bz component field Bz component(red); (g) IMF (red); clock ( angle;g) IMF ( hclock) ion numberangle; ( density;h) ion number (i) the ratio density; of the (i ion) the to electronratio of temperature;the ion to electron (j) ion temperature;differential energy (j) ion flux differential (0.01–30 energy keV). flux (0.01–30 keV).

Atmosphere 2020, 11, 831 10 of 13

Therefore, these two CDPSs are mainly caused by KHI that occurs at LLBL. The cross-correlation analysis suggests that it takes 3.7–5.9 h for solar wind ions to enter the plasma sheet through the pathway, which is in agreement with previous studies [10]. Fujimoto et al. [56] presented a CDPS event where cold ion populations originate from the magnetosheath via LLBL. In the CDPS, these low-energy electrons mainly appear in the parallel and anti-parallel directions, showing the signature of a cigar distribution. In our events, one can observe a similar phenomenon as well (see Figures2g and5g). Wang et al. [ 41] presented a magnetopause crossing at LLBL and found that the Ti/Te was conserved. In our study, however, the Ti/Te in the deeper plasma sheet was not the same as that in the solar wind. For the first event, Ti/Te in the plasma sheet (~9) was larger than that in the solar wind (~4) due to the existence of hotter ions in the plasma sheet, while the ratio of the cold ion to electron temperature in the plasma sheet (~4) was similar to Ti/Te in the solar wind (~4) due to the fact that these cold ions came from the solar wind. For the second event, both Ti/Te (~15) and the cold ion to electron temperature (~5) in the plasma sheet were smaller than Ti/Te in the solar wind (~30), possibly owing to the turbulent plasma environment in the magnetosheath when the solar wind near the LLBL propagated to the downstream ARTEMIS. Thus, we guess that Ti/Te near the LLBL may be ~5. The CDPSs have effects on the dipolarization fronts (DFs) dynamics. The cold and dense plasma can alter the plasma density gradients near the dipolarization fronts [20,63] and thus affect the density-driven instabilities [21,22,64], electric field structure [23], and wave-particle interactions [62] near the DFs. It also possibly affects the growth and decay of the DF. In the first event, MMS detected two DFs (Figure2a) embedded in an earthward ions flow burst (Figure2f). Around these two DFs, the plasma number density varied dramatically, suggesting that the plasma density gradients near the DFs may be inconsistent with that near the normal DF. Further analyses of plasma density gradients near these two DFs remain to be conducted in future studies.

Author Contributions: Y.Y. gave the basic idea of the paper. Y.Y. and Z.C. designed the experiments and analysis tools, Y.Y., Z.C., and F.C. performed the experiments, and analyzed the data; Y.Y. and Z.C. participated in discussion and interpretation of the data. Y.Y. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (grant no. 41821003 and 41874188). Acknowledgments: We thank the MMS team and ARTEMIS team for providing the data for this study. MMS data and ARTEMIS data are available at https://lasp.colorado.edu/mms/sdc/public/ and https://cdaweb.sci.gsfc. nasa.gov/index.html/, respectively. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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