2D Plasma Sheet Ion Density and Temperature Profiles for Northward and Southward IMF Simon Wing and Patrick T

GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 9, 1307, 10.1029/2001GL013950, 2002

2D plasma sheet ion density and temperature profiles for northward and southward IMF Simon Wing and Patrick T. Newell The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA Received 21 August 2001; revised 5 December 2001; accepted 14 December 2001; published 8 May 2002.

[1] We previously developed a method for inferring plasma sheet Using observations from the same satellite, Terasawa et al. ion temperature and density from ionospheric observations. Using [1997] show that during periods of northward IMF plasma sheet this method and DMSP observations for 1992, we present 2D ions are on average denser and colder and there are temperature profiles of the equatorial plasma sheet ion density and temperature minima and density maxima near the dawn and dusk flanks. for southward and northward IMF for b2i > 64 or Kp < 3. However, because of limited spatial coverage, these and other similar results from in situ observations were obtained by During periods of northward IMF, cold dense ions can be found averaging over a large region, blurring the spatial extent of these plentifully along the plasma sheet flanks. However, during periods cold dense ions. Recently Fairfield et al. [2000] and Otto and of southward IMF, the presence of these cold dense ions is Fairfield [2000] present a Geotail event near the dusk flank noticeably diminished, especially at the dusk flank where the during a period of northward IMF and an MHD simulation of the density peak is less discernable. These cold dense ions have been same event. Based on their detail analysis, these authors argue for previously interpreted in terms of magnetosheath ion entry into the Kelvin-Helmhotz (KH) instability as an important mechanism for plasma sheet. Our result suggests that any mechanism proposed to bringing magnetosheath ions from the dusk flank into the plasma transport magnetosheath ions from dusk LLBL to the plasma sheet sheet during periods of strongly northward IMF. along the dusk flank during periods of northward IMF would have [5] Motivated by these results, this study investigates the to be able to do so efficiently over a large spatial scale. INDEX effect of the IMF Bz in the 2D equatorial plasma sheet ion n and T profiles with the method developed in Wing and Newell TERMS: 2764 Magnetospheric Physics: Plasma sheet; 2748 [1998]. Magnetospheric Physics: Magnetotail boundary layers; 2744 Magnetospheric Physics: Magnetotail; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions 2. Data Set [6] For this study, we used data obtained from the SSJ4 instru- 1. Introduction ment onboard DMSP satellites F8, F9, F10, and F11 for the entire year of 1992. In 1992, there were at least three DMSP satellites in [2] The Earth’s magnetotail plasma is highly variable spatially operation simultaneously (F8, F10, and either F9 or F11); for one as well as temporally. The 1D plasma sheet temperature (T), month, March, all four were in operation. Because of its upward density (n), and pressure (p) profiles along the midnight meridian pointing and limited pitch angle resolution, DMSP SSJ4 measures or along dawn-dusk are well known [e.g., Angelopoulos et al., only highly field-aligned precipitating particles at an altitude of 1993; Spence et al., 1989; Huang and Frank, 1994; Baumjohann roughly 835 km. et al., 1989]. These plasma sheet parameters have strong depend- [7] NASA NSSDC OMNIWeb provides the IMP-8 hourly ence on the geomagnetic activity [e.g., Huang and Frank, 1994; average solar wind data. Baumjohann et al., 1989, 1990] and on the solar wind and IMF [e.g., Borovsky et al.,1998;Terasawa et al., 1997]. These parameters are obtained from in situ measurements taken by 3. Method for Inferring Plasma Sheet Ion T, n, satellites that sampled only relatively small portions of the and p from Ionospheric Observations magnetotail. As a result, these studies typically have to average [8] The method for inferring the plasma sheet ion T, n, and p over large areas that are in tens to hundreds of cubic Earth radii 3 from the DMSP SSJ4 measurements has been described fully (R ). E elsewhere [Wing and Newell, 1998, 2000] and is briefly summar- [3] We have previously developed a method to infer the ized here. plasma sheet p, n, and T from ionospheric observations. Iono- [9] It has been known for more than two decades that spheric polar orbiting satellites move rapidly, with a typical energetic ions observed in the topside ionosphere are isotropic orbital period near 100 min, allowing them to image the plasma above a certain latitude [e.g., Bernstein et al., 1974]. Likewise, an sheet 25 times per day. Plasma in the plasma sheet has been overwhelming number of in situ observations in the plasma sheet observed to be nearly isotropic [e. g., Spence et al., 1989; Kistler tailward of 8–10 R indicate that plasma is nearly isotropic, et al., 1992; Huang and Frank, 1994] and as a result p, n, and T E irrespective of activity levels [e.g., Kistler et al., 1992; Spence are conserved along the field line. Because of the abundance of et al., 1989; Huang and Frank, 1994]. As a result, n, T, and p are the ionospheric observations, inferred 2-D spatial profiles of approximately conserved along the magnetic field line. The ions plasma sheet ion temperature, density, and pressure could be maintain their isotropy by pitch angle scattering in the tail current constructed at unprecedented fine spatial resolution [Wing and sheet [Lyons and Speiser, 1982]. Nearer to Earth (typically Newell, 1998]. earthward of 8–10 R , although this distance varies with [4] Magnetosheath ion entry into the plasma sheet has been E magnetic activity), the field line becomes more dipolar, and the inferred from observations of cold dense ions in the plasma sheet pitch angle scattering ceases. As a result, our method does not flanks in several Geotail passes [e.g., Fujimoto et al., 1998]. work in this region. Newell et al. [1996, 1998] have developed a computer algorithm for identifying the equatorward isotropy Copyright 2002 by the American Geophysical Union. boundary in the ionosphere (b2i) or equivalently the earthward 0094-8276/02/2001GL013950$05.00 boundary of the central plasma sheet. Sergeev et al. [1993]

21 - 1 21 - 2 WING AND NEWELL: 2D PLASMA SHEET ION DENSITY AND TEMPERATURE PROFILES

Figure 1. Two-dimensional equatorial profiles of plasma sheet ion (a) pressure, (b) temperature, and (c) density during quiet time, normalized b2i = 66–68, average Kp 1.5 (from plates 1, 3, and 4 of Wing and Newell [1998]). The ion density profile for combined 2 quiet and moderate time, normalized b2i > 64 or Kp < 3 is shown in (d). Each point is averaged over 1 Â 1RE regions. Some regions are left blank either because of insufficient data points or because plasma is anisotropic (in the region closer to the Earth). showed that the ion isotropy boundary (or b2i) highly correlates selected. This takes into account ions outside the detectors’ with the tail inclination angle measured in the same local time energy range. sector by GOES geosynchronous satellites (correlation coeffi- cient = 0.9). Therefore b2i can be used to modify and improve ionosphere-magnetosphere T89 magnetic field model [Tsyga- 4. The Quiet Time 2D Plasma Sheet T, n, and nenko, 1989] tail mapping [Sergeev et al., 1993] and has been p Profiles adapted into our method. Because b2i has a local time variation, it is normalized to its midnight value. [11] The inferred 2D magnetically quiet time ion p, T, and n 2 [10] Electron acceleration events are excluded from the data profiles in the neutral sheet averaged in 1 Â 1RE bins are shown in because they distort the plasma sheet ion spectra, resulting in Figures 1a–1c respectively. The data in these figures have an distributions that do not represent those in the plasma sheet. average Kp 1.5 and b2i = 66–68. As shown in Figures 1a and Instead of computing moment, which is widely used, each ion 1b, pressure and temperature are higher in the dusk-premidnight spectrum is fitted to distribution functions (one-component Max- than in the postmidnight-dawn sector in the near Earth region, wellian, two-component Maxwellian, and k) and the best fit is between 8–10 RE from the Earth. This result is consistent with WING AND NEWELL: 2D PLASMA SHEET ION DENSITY AND TEMPERATURE PROFILES 21 - 3

Figure 2. The IMF Bz dependence of the plasma sheet ion density and temperature constructed from the same data set used in Figure 1d, b2i > 64 or Kp < 3. The ion density 2D profiles for the northward and southward IMF are shown in (a) and (b) respectively and their corresponding temperature profiles are shown in (c) and (d) respectively. the E Â B and the gradient/curvature motions of ions originating dawn LLBL ions undergoing E Â B and gradient/curvature drifts, from the deep tail and low-latitude boundary layer (LLBL). The shows that (a) pressure and temperature have a maximum in the E Â B convection moves ions of all energies Earthward, but the near-Earth dusk sector and (b) along the dawn flank, the temper- curvature and gradient duskward (or westward) drift separates ature has a minimum and the density has a maximum [Spence and the ions by energy. Colder ions convect Earthward with little Kivelson, 1993]. The model does not include the dusk LLBL ion westward/duskward displacement, but hotter ions drift further source, which explains the absence of the temperature minimum duskward, resulting in a higher temperature and pressure in the and the density maximum in the dusk flank. Figures 1b and 1c dusk-premidnight sector at approximately 8–10 RE away from the suggest that LLBL ions may enter the plasma sheet along the dusk Earth. Figures 1b and 1c show that the temperature is colder and flank, which is the main topic in the next two sections of this paper. density higher near both flanks, where most likely there is mixing The plasma pressure also has a maximum near midnight that may of colder magnetosheath like ions from LLBL and hotter ions from have resulted partly from weaker magnetic field and pressure the plasma sheet [e.g., Fujimoto et al., 1998; Eastman et al., 1985; balance. In addition, bursty bulk flows (BBFs) which occur more Spence and Kivelson, 1993]. The Finite-width magnetotail con- frequently near the midnight meridian may also contribute to the vection model calculation, which takes into account deep tail and higher pressure and temperature near midnight meridian [e.g., 21 - 4 WING AND NEWELL: 2D PLASMA SHEET ION DENSITY AND TEMPERATURE PROFILES

Angelopoulos et al., 1993]. Comparisons of Figures 1c with in [14] Acknowledgments. The DMSP SSJ4 instrument was designed situ measurements [e.g., Huang and Frank, 1994; Baumjohann and built by Dave Hardy and colleagues at the AFRL. Fred Rich has been et al., 1989, 1990; Lennarsson, 1992; Lennartsson and Shelley, helpful in our acquiring this data, as has the World Data Center in Boulder, Colorado. NASA NSSDC OMNIWeb provided the IMP-8 solar wind data. 1986] and model calculations (Spence and Kivelson [1993] and The work of SW was supported by NSF Space Weather Grant ATM- Wang et al. [2001]) are in general very good [Wing and Newell, 9819705 and NASA Grant NAG5-10971; the work of PTN was supported 1998; Wang et al., 2001]. by AFOSR Grant F49620-00-1-0172.

References Angelopoulos, V., C. F. Kennel, F. V. Coroniti, R. Pellat, H. E. Spence, 5. IMF Bz Effect on Plasma Sheet Ion n and T M. G. Kivelson, R. J. Walker, W. Baumjohann, W. C. Feldman, J. T. [12] We have previously separated the plasma sheet ion n, T, Gosling, and C. T. Russell, Characteristics of ion flow in the quiet state of the inner plasma sheet, Geophys. Res. Lett., 20, 1711–1714, 1993. and p by b2i or magnetic activity [Wing and Newell, 1998]. Bernstein, W., B. Hultqvist, and H. Borg, Some implications of low altitude Motivated by recent results [e.g., Fairfield et al., 2000; Otto and observations of isotropic precipitation of ring current protons beyond the Fairfield, 2000], we searched for the IMF Bz dependence on the plasmapause, Planet. Space Sci., 22, 767, 1974. plasma sheet ion n and T. In order to increase the number of Baumjohann, W., G. Paschmann, and C. A. Cattell, Average plasma proper- points, we included data for quiet and moderate time, normalized ties in the central plasma sheet, J. Geophys. Res., 94, 6597–6606, 1989. b2i > 64 or Kp < 3. Figure 1d shows the 2D equatorial density Baumjohann, W., G. Paschmann, and H. Luhr, Pressure balance between lobe and plasma sheet, Geophys. Res. Lett., 17, 45–48, 1990. profile when data for all IMF Bz. Figure 1d shows that the density is Borovsky, J. E., M. F. Thomnsen, and R. C. Elphic, The driving of the plasma generally higher than the quiet time density, in agreement with sheet by the solar wind, J. Geophys. Res., 103, 17,617–17,639, 1998. previous in situ measurements [e.g., Baumjohann et al., 1989]. Eastman, T. E., L. A. Frank, and C. Y. Huang, The boundary layers as the Apart from the higher density in Figure 1d, Figures 1d and 1c are primary transport regions of the Earth’s magnetotail, J. Geophys. Res., similar in that both show the presence of density peaks at both 90, 9541–9560, 1985. the dusk and dawn flanks. However, starkly contrasting pictures Fairfield, D. H., A. Otto, T. Mukai, S. Kokubun, R. P. Lepping, J. T. Steinberg, A. J. Lazarus, and T. Yamamoto, Geotail observations of the of the plasma sheet flanks emerge when the data in Figure 1d are Kelvin-Helmholtz instability at the equatorial magnetotail boundary for separated by the sign of the 2-component of IMFs. This was done parallel northward fields, J. Geophys. Res., 105, 21,159–21,173, 2000. using IMP-8 hourly average IMF data. The results are shown in Fujimoto, M., T. Terasawa, T. Mukai, Y. Saito, T. Yamamoto, and S. Ko- Figures 2a and b for northward and southward IMF, respectively. kubun, Plasma entry from the flanks of the near-Earth magnetotail: Geo- As previously reported, the plasma sheet ion density is higher tail observations, J. Geophys. Res., 103, 4391–4408, 1998. Huang, C. Y., and L. A. Frank, A statistical survey of the central plasma during periods of northward IMF than southward IMF [e.g., sheet, J. Geophys. Res., 99, 83–95, 1994. Terasawa et al., 1997], but the profile is presented here in more Kistler, L. M., E. Mobius, W. Baumjohann, G. Paschmann, and D. C. detail in 2D. During periods of northward IMF, density peaks Hamilton, Pressure changes in the plasma sheet during substorm injec- appear along the plasma sheet dawn and dusk flanks. However, tions, J. Geophys. Res., 97, 2973–2983, 1992. during periods of southward IMF, in the dawn flank the density Lennartsson, W., and E. G. Shelley, Survey of 0.1- to 16-keV/e plasma peak is smaller and narrower (in the y direction) and in the dusk sheet ion composition, J. Geophys. Res., 91, 3061–3076, 1986. Lennarsson, W., A scenario for solar wind penetration of Earth’s magnetic flank it is even less discernable. During periods of northward tail based on ion composition data from ISEE 1 spacecraft, J. Geophys. IMF, ion temperature has minima along the dawn and dusk flanks Res., 97, 19,221–19,238, 1992. as shown in Figure 2c. However, during periods of southward Lyons, L. R., and T. W. Speiser, Evidence for current sheet acceleration in IMF, the temperature minima are smaller and narrower, as shown the geomagnetic tail, J. Geophys. Res., 87, 2276–2286, 1982. in Figure 2d. Newell, P. T., Y. I. Feldstein, Y. I. Galperin, and C.-I. Meng, Morphology of nightside precipitation, J. Geophys. Res., 101, 10,737–10,748, 1996. Newell, P. T., V. A. Sergeev, G. R. Bikkuzina, and S. Wing, Characterizing the state of the magnetosphere: Testing the ion precipitation maxima 6. Discussion and Summary latitude (b2i) and the ion isotropy boundary, J. Geophys. Res., 103, 4739–4745, 1998. [13] For the first time, the IMF Bz dependence of the plasma Otto, A., and D. H. Fairfield, Kelvin-Helmholtz instability at the magneto- sheet ion n and T profiles is exhibited in fine spatial detail in 2D. tail boundary: MHD simulation and comparison with Geotail observa- The plasma sheet ion n and T profiles generally differ for the tions, J. Geophys. Res., 105, 21,175–21,190, 2000. northward and southward IMF cases, but the most remarkable Sergeev, V. A., M. Malkov, and K. Mursula, Testing the isotropic boundary contrast occurs in the density profiles along the plasma sheet dusk algorithm method to evaluate the magnetic field configuration in the tail, flank. During periods of northward IMF, ions are colder and denser J. Geophys. Res., 98, 7609–7620, 1993. Spence, H. E., M. G. Kivelson, R. J. Walker, and D. J. McComas, Magneto- along both flanks of the plasma sheet, i.e. adjacent to the magnetospheric plasma pressures in the midnight meridian: Observations from 2.5 pause. However, during periods of southward IMF, this density to 35 RE, J. Geophys. Res., 94, 5264–5272, 1989. peak is smaller along the dawn flank, and it is barely discernible Spence, H. E., and M. G. Kivelson, Contributions of the low-latitude along the dusk flank. Likewise, the temperature minima are smaller boundary layer to the finite width magnetotail convection model, J. Geo- and narrower at the flanks, but the dawn-dusk asymmetry appears phys. Res., 98, 15,487–15,496, 1993. not to be as strong as in the density profile. This result is consistent Terasawa, T., et al., Solar wind control of density and temperature in the near-Earth plasma sheet: WIND/GEOTAIL collaboration, Geophys. Res. with the previous Geotail observations [Terasawa et al., 1997], but Lett., 24, 935–938, 1997. here the spatial extent of these cold dense ions is shown in detail in Tsyganenko, N. A., A magnetospheric magnetic field model with a warped 2D. The presence of the cold dense ions along the plasma sheet tail current sheet, Planet Space Sci., 37, 5–20, 1989. flanks has been interpreted as a strong indication of magnetosheath Wang, C.-P., L. R. Lyons, M. W. Chen, and R. A. Wolf, Modeling the quiet ion entry [e.g., Fujimoto et al., 1998]. Consequently, the result here time inner plasma sheet protons, J. Geophys. Res., 106, 6161 –6178, 2001. indicates that far more magnetosheath ions enter from the dusk Wing, S., and P. T. Newell, Central plasma sheet ion properties as inferred LLBL during periods of northward IMF than during periods of from ionospheric observations, J. Geophys. Res., 103, 6785–6800, 1998. southward IMF. Otto and Fairfield [2000] and Fairfield et al. Wing, S., and P. T. Newell, Quiet time plasma sheet ion pressure contribu- [2000] demonstrate that KH instability can transport massive tion to Birkeland currents, J. Geophys. Res., 105, 7793–7802, 2000. magnetosheath ions effectively across the magnetopause dusk flank. Other mechanisms may also be possible and need to be ÀÀÀÀÀÀÀÀÀÀÀ investigated. However, the result here indicates that any such S. Wing and P. T. Newell, The Johns Hopkins University Applied Physics mechanism would have to be able to transport ions efficiently Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland, USA. (simon. over a very large spatial scale. [email protected]; [email protected])