1 Microburst cusp ion precipitation
2 observed with Reimei 3 4 1 2 3 4,5 5 Y. Ebihara, Y. Miyoshi, K. Asamura, and M. Hirahara 6 7 8 1. Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 9 464-8601, Japan 10 2. Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 11 Aichi, 464-8601, Japan 12 3. ISAS/JAXA, 3-1-1 Yoshinodai, Sagamihara, 229-8510, Japan 13 4. Rikkyo University, 3-34-1, Nishi-Ikebukuro, Toshima-ku, Tokyo, 171-8501, Japan 14 5. Now at Department of Earth and Planetary Science, Graduate School of Science, The University 15 of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 16 17 18 19 20 21 KEYWORDS: Cusp, Small-scale ion injection, Topside ionosphere 22 RUNNING TITLE: MICROBURST CUSP ION PRECIPITATION 23
24
1 25 Abstract.
26 At an altitude of 610-670 km, the Japanese satellite Reimei was the first to discover a new
27 type of ion injections into the topside ionosphere in or in the vicinity of the cusp. These ion
28 injections, called Microburst Cusp Ion Precipitation (MCIP), were spatially embedded into
29 the dominant precipitation of magnetosheath-like particles. They were observed in
30 approximately one third of all cusp traversals, and were characterized by a relatively low
31 energy (with less than a few hundred electron-volts) and relatively short–lived nature (with
32 a typical lifetime of 1 to 2 seconds). The characteristic energy decreases with time (Type 1),
33 and sometimes does not clearly exhibit energy-time dispersion (Type 3). Only one event
34 exhibits the characteristic energy increasing with time (Type 2). Applying the time-of-flight
35 model (in which higher energy ions arrive at the observation point first) and the velocity
36 filter model (in which the characteristic energy decreases with distance from a source field
37 line under the influence of perpendicular drift motion), we estimated the source distance
38 from Reimei. This paper proposes that a localized electric field, probably associated with
39 inertial Alfvén waves and/or ionospheric Alfvén resonator, could have been generated at
40 short intervals in time or in isolated regions at an altitude less than 3,000 km, which results
41 in MCIPs accompanied with energy-time and energy-pitch angle dispersions observed at an
42 altitude of 610 to 670 km. 43
2 44 1. Introduction 45 46 The polar cusp is characterized by precipitation of intense electrons (approximately 100
47 eV) and ions (a few hundred eVs to a few keVs) that originate from the magnetosheath
48 (solar wind) [e.g., Woch and Lundin, 1992; Onsager et al., 1993; Newell and Meng, 1995].
49 The dominant ions often exhibit energy-latitude dispersion due to the velocity filter effect,
50 whereby low energy ions tend to drift away from an original field line under the influence
51 of the EB× drift before arriving at the observation point [Shelley et al., 1976; Reiff et al.,
52 1977]. 53 54 Currently, the cusp ions are thought to be a mixture of ions originating from distinct sources.
55 At high altitudes, Chen and Fritz [2001] found low charge state oxygen ions with energy of
56 about 100 keV, which are thought to be of the ionosphere origin. At middle altitudes,
57 observations show that low energy ions, with energies less than about a few hundred eVs,
58 are often observed as ion conics or upgoing ion beams, which indicate that they are locally
59 accelerated [e.g., Kremser and Lundin 1990; Miyake et al., 1993; Pfaff et al., 1998; Moore
60 et al. 1999; Su et al., 2001; Andersson et al. 2002; Bouhram et al. 2002, 2003, 2004;
61 Bogdanova et al., 2004]. Based on data from the FAST satellite, some of the upgoing ion
62 beams appear simultaneously with bursts of downgoing electrons, indicating that they are
63 accelerated by an upward-directed electric field parallel to the magnetic field [Pfaff et al.,
64 1998]. Furthermore, narrow beams of upgoing electrons are commonly seen in the cusp
65 [Burch et al., 1983]. The upgoing electrons vanish when upgoing ions are observed. The
66 same occurs with upgoing ions, which vanish when upgoing electrons are observed. This
67 result implies that polarity of the parallel electric field varies with respect to time. At low
68 altitude (~1,000 km or lower), locally accelerated ions are rarely reported. One of those few
69 is Keith et al. [2001] who showed upgoing ion fluxes in the 3-100 eV energy range within
70 the cusp on the basis of data from the Astrid-2 satellite at an altitude of 1,000 km. Thus,
3 71 downgoing low-energy and bursty ions have not generally been recognized [Yamauchi et
72 al., 2001; Andersson et al., 2002]. 73 74 In this paper, a new class of cusp ion precipitation observed with the Reimei satellite at an
75 altitude of 610 to 670 km is described. This ion precipitation has been observed to occur at
76 a short interval of 1-2 s and is often accompanied by energy-time and energy-pitch angle
77 dispersion features. 78 79 80 2. Instrumentation 81
82 The Reimei satellite was launched on August 23, 2005 into a Sun-synchronous orbit with
83 an altitude of about 610-670 km and an inclination of 97.8°. The Reimei had top-hat type
84 electrostatic analyzers (EISA) [Asamura et al., 2003] that are capable of detecting electrons
85 and ions in the energy range between 10 eV/q and 12 keV/q. It takes 40 ms to complete a
86 32-step energy scan from 10 eV/q to 12 keV/q. EISA has 30 sectors to simultaneously
87 cover a field of view of 300°. The remaining field of view of the detector is obstructed by
88 the solar cell paddles of the satellite. In general, the count rates of the ion sensor are
89 relatively small. In order to improve count statistics, the sum of count rates for the interval
90 of 160 ms was used to illustrate the ion data in this paper. The Reimei satellite does not
91 carry DC and AC magnetic/electric field instruments. So far, transverse measurements of
92 particles over the cusp region were performed in the Northern Hemisphere only. Thus, the
93 data acquired in the Northern Hemisphere is focused on.
94 95 96 3. Observations 97
4 98 Figure 1 shows a sample Reimei measurement as it crossed the northern cusp during
99 08:03:00-08:03:40 UT on October 28, 2005. Reimei was located at 11.7-11.8 hrs magnetic
100 local time (MLT), at invariant latitudes (ILATs) from 75.4° to 77.5°, and at an altitude of
101 about 633-634 km. Data from the ACE satellite indicated that the interplanetary magnetic
102 field (IMF) was about (-1, 0, -3) nT in GSM coordinates, and that the solar wind velocity
103 was approximately 440 km·s-1 with a density of about 7 cm-3. The electron spectrum
104 includes bursts of accelerated electrons with peak energies at a few hundred eVs, which
105 may have been accelerated by the inertial Alfvén waves [e.g., Andersson et al., 2002]. The
106 ion spectrum shows a continuous and broad structure of downgoing ions with the
107 characteristic energy decreasing from a few keVs to about hundred eVs as Reimei traveled
108 from the low to high latitude. The energy-latitude dispersion of the dominant ions can be
109 interpreted by a velocity filter effect under the poleward E×B convection electric field
110 [Shelley et al., 1976; Reiff et al., 1977] during a southward IMF. The dominant ions are
111 thought to be of magnetosheath origin because the ion flux is less dependent on the pitch
112 angle, which implies that the entry region should be located far (≥10 RE) from the 113 low-altitude Reimei satellite [Reiff et al., 1977; Burch et al., 1982].
114
115 The white arrows in Figure 1 show the bursty injections of ions with energy less than about
116 a few hundred eVs in the ion spectrum. The characteristic energy is less than that of the
117 magnetosheath-like precipitation, and decreases rapidly with time. The lifetime of each
118 bursty injection is only about 1-2 s. As for the first two bursty injections, the characteristic
119 energy decreases with time. These bursty ion injections are referred to as “Type 1
120 Microburst Cusp Ion Precipitation” (Type 1 MCIP). The latter two do not exhibit dispersion
121 clearly. These ones will be referred to as “Type 3 MCIP.”
122
123 Figure 2 shows a close-up view of the MCIPs. It is clearly seen that the continuous and
124 broad structure of downgoing ions with energy greater than 1 keV is not affected by the
5 125 appearance of the MCIPs. The down-going electrons exhibit highly structured variation in
126 time, which is consistent with the Freja observation [Yamauchi et al., 2001]. There seems
127 no correlation between the structured electron variation and the appearance of the MCIPs.
128
129 Figure 3 shows successive snapshots of the energy vs. pitch angle distribution of ions with
130 an interval of 200 ms. The interval corresponds to the one of the leftmost Type 1 MCIP
131 shown in Figure 1. At the beginning of the MCIP, the characteristic energy is about 100 eV
132 with a pitch angle of 0°, which is downward. For a pitch angle of 60°, the characteristic
133 energy was about 300-400 eV. As time proceeds, the characteristic energy rapidly decreases
134 with time for all the pitch angles, and the MCIP vanishes by 08:03:02.33 UT.
135
136 Figure 4 shows the spectra of the downgoing electrons and ions during the next cusp
137 crossing. The characteristic energy of the dominant cusp ions decreases with latitude, which
138 is an indication of a southwardly directed IMF. According to ACE observations, the IMF
139 was directed southward. At least two MCIPs, denoted by white arrows, are clearly
140 identified in the equatorward portion of the dominant precipitation of magnetosheath-like
141 particles. It should be noted that the characteristic energy of the first MCIP seems to
142 increase with time, which is opposite to the trend observed for the MCIP shown in Figure 1.
143 This reversed type MCIP will be referred to as “Type 2 MCIP.” Of note, a Type 1 MCIP
144 appeared within 4 s of the observation of the Type 2 MCIP. The count rate of this MCIP is
145 much higher than the detector-noise count rate, and this MCIP exhibits a clear energy-pitch
146 angle dispersion as is shown below. Thus, this MCIP is most likely real. However, we admit
147 that it is not very clear whether if the characteristic energy increases with time.
148
149 Figure 5 indicates that MCIPs were observed during which the characteristic energy of the
150 dominant precipitation of the magnetosheath-like ions increased with latitude. The energy
151 dispersion of the magnetosheath-like ions can be interpreted by a velocity filter effect under
6 152 the equatorward E×B convection electric field during the northward IMF. In fact, the ACE
153 satellite observed a northwardly oriented IMF. Several microburst injections of ions, which
154 are not accompanied by energy as a function of time dispersion, are present in the spectrum
155 of the downgoing ions. It can be determined that MCIPs appear in the cusp region
156 regardless of the orientation of the IMF.
157
158 A total of 46 cusp crossings of Reimei during the period from November 2005 to February
159 2007 were identified. MCIPs were observed in 15 of the 46 cusp traverses (about 1/3 of the
160 all cusp traverses). In the most cases, they are classified into Type 1 or Type 3 MCIP. Only
161 one was classified into Type 2 during the entire time period investigated. The most
162 important characteristics of MCIPs can be summarized as follows:
163 1. MCIPs appear concurrently with the dominant precipitation of magnetosheath-like
164 particles.
165 2. MCIPs appear regardless of the orientation of the IMF.
166 3. Their characteristic lifetime is about 1-2 s.
167 4. Their characteristic energy is less than that of the dominant magnetosheath-like ions
168 (less than about hundred keVs).
169 5. In general, their characteristic energy increases with the pitch angle from 0° to 90°. As
170 for many of Type 3 MCIPs, the count rate is too faint to show the pitch angle
171 dispersion clearly.
172 6. Energy-time dispersion was occasionally observed. These contain the classes that the
173 characteristic energy either rapidly decreases with time (Type 1) or increases with time
174 (Type 2).
175 7. MCIPs tend to appear concurrently with changes in the characteristic energy of
176 magnetosheath-like ions, but there is not clear one-to-one correspondence between
177 them.
178
7 179 180 4. Distance to the source 181 182 We first attempted to calculate the source distance using the energy-time dispersion feature 183 based on the time-of-flight (TOF) model. Figure 2c shows the inverse velocity as a function 184 of time of the downgoing ions. Solid red lines are drawn to fit the upper and lower 185 boundary of the Type 1 dispersion. If there is no parallel electric field between the source 186 and Reimei, the slope of the red line will give an estimate of the source distance from
187 Reimei. For this particular case, the source distance D|| is estimated to be 50-210 km (13-53 188 km) above Reimei when the ions are purely composed of H+ (O+). This is consistent with 189 the past observation that Freja did not clearly observe downgoing low-energy ions at 1,700 190 km altitude over the cusp [Yamauchi et al., 2001; Andersson et at al., 2002]. Note that the 191 estimated distance depends on the ion composition which was not resolved by Reimei. 192 193 Next, the energy-pitch angle dispersion of the MCIPs observed by Reimei was used to
194 estimate the source distance based on the TOF model. If the quasi-steady state is valid and
195 there is no parallel electric field between the source si and the observation point so, the 196 transit time t between them is given as [Burch et al., 1982]
si ds t = ∫ vs() s0 || 197 si mds (1) = , ∫ 2W B()s s0 2 1sin− α0 B0
198 which yields
2 ⎛⎞ ⎜⎟ mdssi 199 Wt(,α )= ⎜⎟, (2) 0 2 ⎜∫ ⎟ 2t 2 Bs() s0 1sin− α ⎜⎟0 ⎝⎠B0
200 and
8 2 ⎛⎞si ds ⎜⎟∫ ⎜⎟s 2 Bs() 0 1sin− α W ()α ⎜⎟0 B 201 0 = ⎜0 ⎟, (3) WD(0)α0|= ⎜⎟| ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠
202 where ds is arc length along a field line, W is the kinetic energy, m is the particle mass, B0 is
203 the magnetic field at the observation point s0, and B(s) is the magnetic field at s, D|| is the
204 distance between si and s0 along a field line, and α0 is the pitch angle at the observation 205 point. Equation (3) implies that the shape of the energy vs. pitch angle curve is independent
206 of mass. It depends only on the geometry of the magnetic field and D||. 207
208 Figure 6a displays the observed energy-pitch angle distribution of ions for a Type 1 MCIP
209 observed at 08:03:01.73 UT on October 28, 2005, which corresponds to the leftmost MCIP
210 in Figure 1. Two white curves are drawn to fit the upper and lower energy boundaries of the
211 MCIP in Figure 1. The upper curve was calculated for a distance D|| of 300 km, and the 212 lower curve was calculated for a distance of 700 km. We used the IGRF 2000 and the
213 Tsyganenko 1996 [Tsyganenko, 1995; Tsyganenko and Stern, 1996] magnetic field models
214 with parameters specified by data derived from the ACE satellite. Good agreement between
215 the observations and computed results suggests that the estimated source distance ranges
216 between 300 km and 700 km above Reimei along a field line. The same method was used
217 to estimate the source distance for the best 5 MCIPs that have a high signal noise ratio. The
218 results suggest that the source of the MCIPs is widely distributed ranging from 300 km to
219 2,500 km above Reimei’s altitude. The source distance based on the pitch angle dispersion
220 is longer than based on the energy-time dispersion for these particular cases. This
221 discrepancy probably comes from the quasi-steady state assumption that was used to derive
222 Eq. (3), indicating that the source could undergo time-varying. It should be emphasized that
223 the TOF model only explains Type 1 MCIPs, whose characteristic energy decreases with
9 224 time. 225 226 The velocity filter (VF) model gives a plausible explanation for any type of energy-time
227 dispersion [e.g., Frahm et al., 1986]. The characteristic energy decreases with time (Type 1)
2 228 when VEBs •×()B >0, and the energy increases with time (Type 2) 2 229 when VEBs •×()B <0, where Vs is the satellite velocity and E is the electric field. The 2 230 energy-time dispersion is absent (Type 3) when VEBs • ()×B =0. In the VF model, a 231 localized source and a strong electric field perpendicular to the magnetic field are needed to
232 result in the formation of MCIPs. It was assumed that the magnetic field lines are
233 equipotential, and that the perpendicular electric potential is independent of the horizontal
234 distance. The displacement perpendicular to the magnetic field is given as:
si DVsdt= () ⊥⊥∫ s0 si Vsds() 235 = ⊥ (4) ∫ vs() s0 || mdsi s = E , 0 2WB ∫ B()s 0 s0 2 Bs() 1− sinα0 B0
236 where E0 is the electric field at the observation point s0, and Vs⊥ ()()/()= EsBs is the
237 local EB× drift speed. The relationship Es()= E0 Bs () B0 was used to satisfy the 238 assumed condition that the field lines are equipotential. Equation (4) yields
2 ⎡⎤si ds ⎢⎥∫ 2 Bs() ⎢⎥s0 Bs() 1− sinα W ()α ⎢⎥0 B 239 0 = ⎢0 ⎥, (5) si W (0)α0 = ⎢⎥ds ⎢⎥∫ Bs() ⎢⎥s0 ⎣⎦⎢⎥
240 which means that the shape of the energy as a function of pitch angle depends only on the
10 241 geometry of the magnetic field and D|| (=si-s0). Figure 6b shows the energy as a function of 242 pitch angle for the Type 2 MCIP shown in Figure 3. The two white curves that were
243 calculated for D|| = 2,000 km based on the velocity filter model are also included.
244 Substituting the parameters W = 80 eV, α0 = 0° and D⊥ = 8 km into Equation (4), it was
245 determined that the required perpendicular electric field at the Reimei altitude s0 was 24 246 mV/m. This simple calculation provides a lower boundary for the perpendicular electric
247 field. If the electric field is localized in a direction parallel to the magnetic field, rather than
248 distributed widely along a field line, a stronger electric field will be needed to explain the
249 observed energy as a function of pitch angle dispersion. 250 251 252 5. Discussion 253 254 5.1 Uniqueness of MCIP 255 256 Past satellite observations that resolve energy, mass and pitch angle have shown that low 257 energy ions (100 eV or less) found in the cusp region are of the ionospheric origin. They 258 tend to appear as conics, beams or non-thermally heated distribution functions [e.g., 259 Kremser and Lundin, 1990; Moore et al., 1999; Andersson et al., 2002; Bouhram et al., 260 2003, 2004]. Almost all these ions are oriented perpendicular to the local magnetic field or 261 upward along the magnetic field line. Downgoing O+ ions were observed by FAST 262 [McFadden et al., 2003] and Freja [Yamauchi et al., 2005], but they are observed rarely. 263 The energy of the downgoing O+ ions was reported to be about ~ keV and less structured. 264 By contrast, MCIP has a characteristic energy less than ~100 eV and they appear bursty 265 with a characteristic lifetime of 1-2 s. Unlike the downgoing O+ and He+ ions observed by 266 FAST [McFadden et al., 2003], multiple energy peaks corresponding to H+ and O+ are not 267 recognized. This probably means that the ions are accelerated by a quasi-electrostatic field. 268 Thus, the MCIPs observed at 610-670 km altitudes are unique and probably have important 269 implications for our understanding of electromagnetic dynamics in the cusp region.
11 270 271 5.2 Possible mechanisms 272 273 Tanaka et al. [2003] found that there is a relationship between the accelerated electrons and
274 decelerated ions in the vicinity of the cusp at altitudes of 500-1,100 km. They attributed this
275 coincidence to the quasi-static upward potential drop that varied between 40 eV and 165 eV.
276 If such a parallel electric potential is impulsively imposed on the dominant ions originating
277 from the magnetosheath, then an ion cluster, which is decelerated and detached from the
278 dominant ions, will propagate downward towards Reimei. This will result in the observed
279 energy as a function of time and pitch angle dispersion due to the TOF effect. However, the
280 time scale of the observed MCIPs, which is 1-2 s, is much shorter than that which Tanaka et
281 al. [2003] showed was required for their mechanism. 282 283 MCIPs can also result from the downward parallel electric field. In this case, ambient ions
284 pre-existing at an altitude of 1,000 to 3,000 km are assumed to be accelerated downward.
285 Potential drops along the field line were determined based on the near-conjugate
286 observation in the cusp of the electron precipitation noted by DE-1, at an altitude of about
287 20,000 km, and DE-2, at an altitude of about 1,000 km [Lin et al., 1986]. The upper limit of
288 the potential drop was estimated to be about 400 V, which is consistent with that expected
289 from the observations of MCIPs. However, the existence of the spatially-confined and
290 short-lived potential drop that could result in MCIPs has not been proven.
291
292 The parallel electric fields discussed above are the primary requirement for the TOF model
293 that could explain the Type 1 MCIP. In order to account for any type of MCIP, the velocity
294 filter model which involves a localized source and a strong perpendicular electric field is
295 preferred. A question is the cause of the transient electric fields that could have existed
296 within 3,000 km above Reimei.
12 297
298 Observations have shown that the mid-altitude cusp is enriched with large-amplitude
299 electric fields in the frequency range of 0.5-3 Hz, which are regarded as the inertial Alfvén
300 waves (IAW) [e.g., Matsuoka et al., 1993]. Such large-amplitude electric fields have been
301 frequently observed in or in the vicinity of the cusp by Akebono at altitudes of 5,000-8,000
302 km [e.g., Miyake et al., 2003], Freja at altitude of 1,500 km [e.g., Stasiewicz et al., 1998],
303 and FAST at altitudes of 2,000-3,000 km [e.g., Ergun et al., 2005]. Chaston et al. [2004]
304 performed a numerical simulation and showed that the ionospheric ions can be accelerated
305 to energies in the region of keVs by the IAW at the auroral zone, though this simulation was
306 performed only for ion outflow. Theoretical studies [e.g., Lysak, 1991] have shown that
307 Alfvén waves can be trapped in a cavity between the conducting ionosphere and the large
308 gradient of the Alfvén speed at about 3,000 km altitude [e.g., Temerin et al., 1986]. Indeed,
309 the cavity, known to be the ionospheric Alfvén resonator (IAR), has been observed at
310 altitudes around 1,700 km [e.g., Grzesiak, 2000; Chaston et al., 2002]. Hirano et al. [2005]
311 identified the IAR in the cusp region from the Akebono observation and detected the
312 large-amplitude electric field at altitude about 2,000 km. The existence of such
313 large-amplitude electric field by the IAW and/or IAR in the cusp region suggests that the
314 source ions of the MCIPs could have been accelerated by them. By assuming that typical
315 energy of 100 eV and typical lifetime of 2 s, the characteristic scale size of MCIPs is
316 estimated to be about 300 km for H+ and 70 km for O+ in the direction of a field line. If
317 IAW or IAR is the case, the characteristic scale size would represent an upper limit of the
318 scale size of the potential structure associated with IAR or IAR.
319
320 Since the Reimei satellite does not carry instruments that measure electric and magnetic
321 fields, it cannot be determined which of the two models, the time-of-flight model or the
322 velocity filter model, is most appropriate. Simultaneous observations of MCIP with Reimei
323 and the EISCAT Svalbard Radar (EISCAT/ESR) will give some clue on the generation
13 324 mechanism of MCIPs. Numerical simulation that solves the kinetic equations of the
325 particles coupled with the self-consistent electromagnetic fields will be useful to verify the
326 possible mechanisms. This simulation will be considered in future projects.
327
328 329 6. Summary 330
331 A new class of cusp ion precipitation, called Microburst Cusp Ion Precipitation (MCIP),
332 was often observed with Reimei in or in the vicinity of the cusp at an altitude of 610 to 670
333 km. This ion precipitation occurred at a short interval of 1-2 s, and in many cases, it was
334 often accompanied by energy-time and energy-pitch angle dispersion features. The
335 time-of-flight analysis and the pitch angle analysis predict that the source was located
336 within 3000 km altitude. The bursty ions are thought to result from a localized electric field
337 probably associated with inertial Alfvén waves and/or ionospheric Alfvén resonator at an
338 altitude less than 3,000 km.
339 340 Acknowledgements 341
342 IMF and solar wind data were provided by Norman Ness (ACE/MFI) and David J.
343 McComas (ACE/SWEPAM) through NASA/GSFC/CDAWeb. The authors thank Nikolai A.
344 Tsyganenko for the empirical magnetic field model. Grateful acknowledgment is due to
345 Hirobumi Saito, Takeshi Sakanoi, Yasumasa Kasaba, Atsushi Yamazaki, Masaki Okada,
346 Yasuyuki Obuchi, and Tomohiro Ino for the successful operation of the Reimei satellite.
347 This study was supported by the Program for Improvement of Research Environment for
348 Young Researchers from the Special Coordination Funds for Promoting Science and
349 Technology (SCF) commissioned by the Ministry of Education, Culture, Sports, Science
350 and Technology (MEXT) of Japan.
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16 411 412 Grzesiak, M. (2000), Ionospheric Alfvén resonator as seen by Freja satellite, Geophys. Res. 413 Lett., 27, 923-926. 414 415 Hirano, Y., H. Fukunishi, R. Kataoka, T. Hasunuma, T. Nagatsuma, W. Miyake, and A. 416 Matsuoka (2005), Evidence for the resonator of inertial Alfvén waves in the cusp topside 417 ionosphere, J. Geophys. Res., 110, A07218, doi:10.1029/2003JA010329. 418 419 Keith, W. R., J. D. Winningham, and O. Norberg (2001), A new, unique signature of the 420 true cusp, Ann. Geophys., 19, 611-619. 421 422 Kremser, G., and R. Lundin (1990), Average spatial distributions of energetic particles in 423 the midaltitude cusp/cleft region observed by Viking, J. Geophys. Res., 95, 5753--5766. 424 425 Lin, C. S., J. L. Burch, and J. D. Winningham (1986), Near-conjugate observations of polar 426 cusp electron precipitation using DE 1 and DE 2, J. Geophys. Res., 91, 11,186-11,202. 427 428 Lysak, R. L. (1991), Feedback instability of the ionospheric resonant cavity, J. Geophys. 429 Res., 96, 1553-1568. 430 431 Matsuoka, A., K. Tsuruda, H. Hayakawa, T. Mukai, A. Nishida, T. Okada, N. Kaya, and H. 432 Fukunishi (1993), Electric field fluctuations and charged particle precipitation in the 433 cusp, J. Geophys. Res., 98(A7), 11,225–11,234. 434 435 McFadden, J. P., C. W. Carlson, R. Strangeway, and E. Moebius (2003), Observations of 436 downgoing velocity dispersed O+ and He+ in the cusp during magnetic storms, Geophys. 437 Res. Lett., 30(18), 1947, doi:10.1029/2003GL017783. 438 439 Miyake, W., T. Mukai, and N. Kaya (1993), On the evolution of ion conics along the field 440 line from EXOS D observations, J. Geophys. Res., 98(A7), 11,127–11,134.
17 441 442 Miyake, W., A. Matsuoka, and Y. Hirano (2003), A statistical survey of low-frequency 443 electric field fluctuations around the dayside cusp/cleft region, J. Geophys. Res., 444 108(A1), 1008, doi:10.1029/2002JA009265. 445 446 Moore, T.E., Lundin, R., Alcayde, D., Andre, M., Ganguli, S. B., Temerin M., and Yau, A. 447 (1999) Source processes in the high-latitute ionosphere, Chapter 2 in "Magnetospheric 448 Plasma Sources and Losses", edited by B.Hultqvis, B., 449 450 Newell, P. T., and C.-I. Meng (1995), Cusp low-energy ion cutoffs: A survey and 451 implications for merging, J. Geophys. Res., 100(A11), 21,943–21,952. 452 453 Onsager, T. G., C. A. Kletzing, J. B. Austin, and H. MacKiernan (1993), Model of 454 magnetosheath plasma in the magnetosphere: Cusp and mantle particles at low-altitudes, 455 Geophys. Res. Lett., 20(6), 479–482. 456 457 Pfaff, R., et al. (1998), Initial FAST observations of acceleration processes in the cusp, 458 Geophys. Res. Lett., 25(12), 2037–2040. 459 460 Reiff, P. H., T. W. Hill, and J. L. Burch (1977), Solar wind plasma injections at the dayside 461 magnetospheric cusp, J. Geophys. Res., 82, 479-491. 462 463 Shelley, E. G., R. D. Sharp, and R. G. Johnson (1976), Satellite Observations of an 464 Ionospheric Acceleration Mechanism, Geophys. Res. Lett., 3(11), 654-656. 465 466 Stasiewicz, K., G. Holmgren, and L. Zanetti (1998), Density depletions and current 467 singularities observed by Freja, J. Geophys. Res., 103(A3), 4251–4260. 468 469 Su, Y.-J., R. E. Ergun, W. K. Peterson, T. G. Onsager, R. Pfaff, C. W. Carlson, R. J. 470 Strangeway (2001), Fast Auroral Snapshot observations of cusp electron and ion
18 471 structures, J. Geophys. Res., 106(A11), 25595-25600, 10.1029/2001JA000093. 472 473 Tanaka, H., Y. Saito, S. Ishii, K. Asamura, and T. Mukai (2003), Simultaneous observation 474 of the electron acceleration and ion deceleration in the dayside high-latitude auroral 475 region, Geophys. Res. Lett., 30(12), 1615, doi:10.1029/2003GL017071. 476 477 Temerin, M., J. McFadden, M. Voehm, C. W. Carlson, and W. Lotko (1986), Production of 478 flickering aurora and field-aligned electron flux by electromagnetic ion cyclotoron 479 waves, J. Geophys. Res., 91, 5769-5792. 480 481 Tsyganenko, N. A. (1995), Modeling the Earth's magnetospheric magnetic field confined 482 within a realistic magnetopause, J. Geophys. Res., 100(A4), 5599-5612. 483 484 Tsyganenko, N. A., and D. P. Stern (1996), A new-generation global magnetosphere field 485 model, based on spacecraft magnetometer data, ISTP Newsl., 6(1), 21. 486 487 Yamauchi, M., L. Andersson, P.-A. Lindqvist, S. Ohtani, J.H. Clemmons, J.-E. Wahlund, L. 488 Eliasson, and R. Lundin (2001), Acceleration signatures in the dayside boundary layer 489 and the cusp, Phys. Chem. Earth, 26, 195-200. 490 491 Yamauchi, M., L. Eliasson, R. Lundin, and O. Norberg (2005), Unusual heavy ion injection 492 events observed by Freja, Ann. Geophys., 23, 535-543. 493 494 Woch, J., and R. Lundin (1992), Magnetosheath plasma precipitation in the polar cusp and 495 its control by the interplanetary magnetic field, J. Geophys. Res., 97(A2), 1421–1430. 496 497 498
19 499 Figure captions 500 501 Figure 1 502 Energy as a function of time for the differential energy flux of electrons and ions from 503 08:03:00 to 08:03:40 UT on October 28, 2005. From top to bottom, the downgoing electron 504 fluxes, trapped electron fluxes, upgoing electron fluxes, downgoing ion fluxes, trapped ion 505 fluxes, and upgoing ion fluxes. The white arrows indicate the microburst cusp ion 506 precipitation (MCIP). The first two are referred as Type 1 MCIP in which the characteristic 507 energy decreases with time, and the latter two are referred as Type 3 in which the 508 characteristic energy do not have an obvious dispersion. Ephemeris information is also 509 included at the bottom of the figure. MLT represents the magnetic local time in hours, ILAT 510 represents the invariant latitude in degrees, FLAT represents the footprint geodetic latitude 511 in degrees, FLON represents the footprint geodetic longitude in degrees, and ALT 512 represents the geodetic altitude in kilometers. 513 514 Figure 2 515 Close-up view of Figure 1. From top to bottom, (a) energy as a function of time for the 516 differential energy flux of downgoing electrons, (b) downgoing ions, and (c) inverse 517 velocity as a function of time for downgoing ion flux are shown (ions are assumed to be 518 purely consist of proton). 519 520 Figure 3 521 Energy as a function of pitch angle for the differential energy flux of ions for the leftmost 522 Type 1 MCIP shown in Figure 1. 523 524 Figure 4 525 Energy as a function of time for the differential energy flux of downgoing electrons and 526 ions from 08:38:50 to 08:39:40 UT on October 28, 2005. The left white arrow indicates the 527 Type 2 MCIP, while the right one indicates the Type 1 MCIP. 528
20 529 Figure 5 530 MCIPs observed during a northward IMF. At least, 6 Type 3 MCIPs are identified. 531 532 Figure 6 533 (a) Energy as a function of pitch angle for the Type 1 MCIP that corresponds to the leftmost 534 one in Figure 1, and (b) the Type 2 MCIP that corresponds to the left one in Figure 4. The 535 white lines drawn in the panel (a) represent the energy as a function of pitch angle 536 relationship for sources at distances of 300 and 700 km from Reimei on the basis of the 537 time-of-flight model. The white lines in the panel (b) represent the relationship for the 538 sources at a distance of 2,000 km from Reimei on the basis of the velocity filter model.
539
21
10000 109
o 1000 108 s str eV 2 100 107 e- 0-30
Energy (eV) 6 10 10 eV/cm 10000 109 o 1000 108 s str eV 2 100 107 e- 60-120
Energy (eV) 6 10 10 eV/cm 10000 109 o 1000 108 s str eV 2 100 107 e- 150-180 Energy (eV) 6 10 10 eV/cm 10000 Type 1 Type 3 o 107 1000 s str eV 6 2 100 10 Ion 0-30
Energy (eV) 5 10 10 eV/cm 10000 o 107 1000 s str eV 6 2 100 10 Ion 60-120 Energy (eV) 5 10 10 eV/cm 10000 o 107 1000 s str eV 6 2 100 10 Energy (eV)
Ion 150-180 5 10 10 eV/cm UT :03:00 :03:05 :03:10 :03:15 :03:20 :03:25 :03:30 :03:35 :03:40 MLT 11.8 11.8 11.8 11.8 11.7 11.7 11.7 11.7 11.7 ILAT 75.4 75.7 75.9 76.2 76.5 76.7 77.0 77.2 77.5 FLAT 79.4 79.6 79.8 80.0 80.2 80.4 80.6 80.8 81.0 FLON 28.7 27.5 26.3 25.0 23.7 22.3 20.8 19.3 17.7 ALT 632.9 633.1 633.2 633.4 633.5 633.7 633.8 633.9 634.1 UT from OCT 28,2005 8:03:00 UT Figure 1
1 (a) 10000 109
o 1000 108 s str eV 2 100 107 e- 0-30
Energy (eV) 6
10 10 eV/cm (b) 10000 Type 1 Type 3
o Type 1 107 1000 s str eV 6 2 100 10 Ion 0-30
Energy (eV) 5 10 10 eV/cm (c) 140
o 120 7 100 10
80 s str eV 60 106 2
Ion 0-30 40 1/V (s/Re) 20 5 10 eV/cm UT :03:00 :03:02 :03:04 :03:06 :03:08 :03:10 MLT 11.8 11.8 11.8 11.8 11.8 11.8 ILAT 75.4 75.5 75.6 75.7 75.8 75.9 FLAT 79.4 79.5 79.6 79.7 79.8 79.8 FLON 28.7 28.3 27.8 27.3 26.8 26.3 ALT 632.9 633.0 633.1 633.1 633.2 633.2 UT from OCT 28,2005 8:03:00 UT Figure 2
2 (1) 08:03:01.53 UT (2) 08:03:01.73 UT (3) 08:03:01.93 UT (4) 08:03:02.13 UT (5) 08:03:02.33 UT (6) 08:03:02.53 UT 10000 7 10 1000 s str eV 6 2 10
Energy (eV) 100 eV/cm 5 10 10 -180 -90 0 90 180 -90 0 90 180 -90 0 90 180 -90 0 90 180 -90 0 90 180 -90 0 90 180 Pitch Angle (deg) Pitch Angle (deg) Pitch Angle (deg) Pitch Angle (deg) Pitch Angle (deg) Pitch Angle (deg) Figure 3
3 (a) 10000 109
o 1000 108 s str eV 2 100 107 e- 0-30
Energy (eV) 6
10 10 eV/cm (b) 10000
o 107 1000 s str eV 6 2 100 10 Ion 0-30
Energy (eV) 5 10 10 eV/cm UT :38:56 :38:57 :38:58 :38:59 :39:00 (c) 10000 109
o 1000 108 s str eV 2 100 107 e- 0-30
Energy (eV) 6
10 10 eV/cm (d) 10000 Type 1 o 107 1000 Type 2 s str eV 6 2 100 10 Ion 0-30
Energy (eV) 5 10 10 eV/cm UT :38:50 :39:00 :39:10 :39:20 :39:30 :39:40 MLT 12.5 12.5 12.5 12.5 12.5 12.5 ILAT 72.3 72.8 73.4 74.0 74.6 75.2 FLAT 75.3 75.9 76.4 76.8 77.3 77.8 FLON 18.7 17.4 16.0 14.5 12.9 11.2 ALT 630.3 630.6 630.9 631.3 631.6 631.9 UT from OCT 28,2005 9:38:50 UT Figure 4
4 (a) 10000 109
o 1000 108 s str eV 2 100 107 e- 0-30
Energy (eV) 6
10 10 eV/cm (b) 10000
o 107 1000 s str eV 6 2 100 10 Ion 0-30
Energy (eV) 5
10 10 eV/cm UT :21:10 :21:11 :21:12 :21:13 (c) 10000 109
o 1000 108 s str eV 2 100 107 e- 0-30
Energy (eV) 6 10 10 eV/cm (d) 10000
o Type 3 107 1000 s str eV 6 2 100 10 Ion 0-30
Energy (eV) 5 10 10 eV/cm UT :21:00 :21:05 :21:10 :21:15 :21:20 :21:25 :21:30 MLT 11.9 11.9 11.8 11.8 11.8 11.8 11.8 ILAT 76.7 77.0 77.2 77.5 77.8 78.0 78.3 FLAT 80.2 80.4 80.6 80.8 80.9 81.1 81.3 FLON 19.4 18.1 16.6 15.1 13.6 12.0 10.3 ALT 641.7 641.8 642.0 642.1 642.2 642.4 642.5 UT from NOV 3,2005 8:21:00 UT Figure 5
5 (a) Type 1 MCIP (b) Type 2 MCIP 10000
7 D||=700 km 10 1000 s str eV 2 6 10 Energy (eV) 100 D||=300 km eV/cm
D||=2000 km
5 10 10 0 90 180 0 90 180 Pitch Angle (deg) Pitch Angle (deg) 08:03:01.73 UT 09:38:57.67 UT OCT 28, 2005 OCT 28, 2005 Figure 6
6