Dawnside Magnetosphere Research Articles

THEMIS observations of compressional Pc5 pulsations in the dawn- and duskside magnetosphere

We present case and statistical studies of compressional Pc5 pulsations observed by the THEMIS spacecraft in the dawn- and duskside magnetosphere from 2007 to 2011. A case study provides evidence for compressional Pc5 pulsations in the dawnside magnetosphere following the arrival of particles injected by a substorm and the subsequent growth of the drift mirror instability on February 27, 2011. The statistical study shows that the Pc5 pulsations occur in a weak or moderately disturbed magnetosphere but rarely develop during strong magnetic activity (i. e., for Dst index below −25 nT). The most striking characteristic of the compressional Pc5 pulsations is their localization in latitude and longitude over many consecutive days. Dawnside Pc5 pulsations occur under quieter conditions, last longer, and attain greater amplitudes than duskside Pc5 pulsations. We attribute compressional pulsations in both sectors to the drift mirror instability, with a warmer, more energized, and gradient-curvature drifting population of substorm-injected ions on the duskside and a colder ExB drifting population of plasma sheet ions on the dawnside.

Structure and dynamics of the Hermean magnetosphere revealed by electron observations from the Mercury electron analyzer after the first three Mercury flybys of BepiColombo

The Mercury electron analyzer (MEA) obtained new electron observations during the first three Mercury flybys by BepiColombo on October 1, 2021 (MFB1), June 23, 2022 (MFB2), and June 19, 2023 (MFB3). BepiColombo entered the dusk side magnetotail from the flank magnetosheath in the northern hemisphere, crossed the Mercury solar orbital equator around midnight in the magnetotail, traveled from midnight to dawn in the southern hemisphere near the closest approach, and exited from the post-dawn magnetosphere into the dayside magnetosheath. We aim to identify the magnetospheric boundaries and describe the structure and dynamics of the electron populations observed in the various regions explored along the flyby trajectories. We derive 4s time resolution electron densities and temperatures from MEA observations. We compare and contrast our new BepiColombo electron observations with those obtained from the Mariner 10 scanning electron spectrometer (SES) 49 years ago. A comparison to the averaged magnetospheric boundary crossings of MESSENGER indicates that the magnetosphere of Mercury was compressed during MFB1, close to its average state during MFB2, and highly compressed during MFB3. Our new MEA observations reveal the presence of a wake effect very close behind Mercury when BepiColombo entered the shadow region, a significant dusk-dawn asymmetry in electron fluxes in the nightside magnetosphere, and strongly fluctuating electrons with energies above 100s eV in the dawnside magnetosphere. Magnetospheric electron densities and temperatures are in the range of 10-30 cm$^ $ and above a few 100s eV in the pre-midnight-sector, and in the range of 1-100 cm$^ $ and well below 100 eV in the post-midnight sector, respectively. The MEA electron observations of different solar wind properties encountered during the first three Mercury flybys reveal the highly dynamic response and variability of the solar wind-magnetosphere interactions at Mercury. A good match is found between the electron plasma parameters derived by MEA in the various regions of the Hermean environment and similar ones derived in a few cases from other instruments on board BepiColombo.

Signatures of Open Magnetic Flux in Jupiter's Dawnside Magnetotail

AbstractJupiter's magnetosphere exhibits notable distinctions from the terrestrial magnetosphere. The structure and dynamics of Jupiter's dawnside magnetosphere can be characterized as a competition between internally driven sunward flow and solar wind‐driven tailward flow. During the prime mission, Juno acquired extensive data from dawn to midnight, sampling the magnetodisc and higher latitude regions. Numerical moments from the Jovian Auroral Distributions Experiment (JADE‐I) plasma (ion) instrument revealed a mid‐latitude region of anticorotational (−vϕ) flow. While the magnitude of the flow is subject to uncertainty due to low count rates in these rarefied regions, we demonstrate in the raw JADE‐I data that the sign of vϕ is a robust measurement. Global Grid Agnostic Magnetohydrodyamics for Extended Research Applications simulations show a similar region of strongly reduced flow in proximity to open field lines. Additionally, we use Jupiter Energetic‐particle Detector Instrument integral moments to determine the Hen+/H+ ratio (where n refers to He+ or He++) and show that a transition to solar wind‐like composition occurs in the same region as the anticorotational flow. We conclude that the global simulations are consistent with the Juno data, where the simulations show a crescent of open magnetic flux that is bounded by the magnetodisc and a closed high‐latitude polar region (nominally the polar cap), which is never observed in the terrestrial magnetosphere. The distinct distribution of open flux in Jupiter's dawnside magnetosphere suggests the significance of planetary rotation and may represent a characteristic feature of rotating giant magnetospheres for future exploration.

Multipoint observations of compressional Pc5-6 pulsations in the dawn side magnetosphere: A case study

We present a case study of compressional Pc5-6 pulsations observed by THEMIS-D and -E in the dawn sector of the magnetosphere following 1100 UT on January 22, 2010. The pulsations were observed during a moderate substorm triggered by the IMF discontinuity and an accompanying change in the solar wind dynamic pressure at 1045 UT. The event can be considered a stormtime Pc5-6 event. We determine the characteristics of the waves, including their velocity, azimuthal wave number m and wavelength. The arrival of particles injected by the substorm caused favorable conditions for the development of the mirror-mode instability and therefore initiated the pulsations. A decrease in the solar wind density and dynamic pressure at 1250 UT was seen in the dayside magnetosheath, corresponding to diminished dayside magnetospheric magnetic fields strengths and terminated the compressional Pc5-6 pulsations. We deduce that the pulsations observed by THEMIS might be internally initiated and externally terminated.

Transport and Acceleration of O+ Ions in Upstream Solar Wind Due To Impact of an IMF Discontinuity: 3D Global Hybrid Simulation

AbstractBased on the predictions of global 3D hybrid simulations, we present a new transport/acceleration path for escaped O+ ions in the upstream solar wind region resulting from the impact of a particular IMF tangential discontinuity (TD) with negative (positive) IMF Bz on the discontinuity's anti‐sunward (sunward) side. For O+ ions escaping to the duskside magnetosheath and with gyro‐radii larger than the TD thickness, when they encounter the TD, they can first go sunward into the upstream solar wind. They then gyrate clockwise to the pre‐noon side and get accelerated within the solar wind region and circulate back to the dawnside magnetosphere. These ions may be accelerated to well within the ring current energy range depending on the solar wind electric field strength. This new transport/acceleration path can bring some of the escaped ions into the inner magnetosphere, thus providing a new mechanism for generating an O+ ring current population.

Periodicities and Plasma Density Structure of Jupiter's Dawnside Magnetosphere

AbstractAbility to quantify variations in magnetic field topology and density within Jupiter's magnetosphere is an important step in understanding the overall structure and dynamics. The Juno spacecraft has provided a rich data set in the dawnside magnetosphere. The recent Grid Agnostic MHD for Extended Research Applications (GAMERA) global simulation study by Zhang et al. (2021, https://doi.org/10.1126/sciadv.abd1204) showed a highly structured plasmadisc with closed magnetic field lines mapped between the outer dawn‐tail flank and the high‐latitude polar region. To test these model predictions, we examined Juno's magnetic field data and electron/energetic particle data to categorize portions of orbits 1–15 into one of three regions based on plasma confinement: the flux pileup region, the intermediate region, and the plasmadisc region. For each region we examined periodicities from magnetic field fluctuations and particle density fluctuations on the 1–10 hr time scale. Periodicities on this time scale could relate to internal (e.g., plasmadisc structure) or external processes (e.g., Kelvin‐Helmholtz vortices). Similar analysis was performed on the GAMERA simulation with the data split into two regions, an outer (150 > R > 60) region and an inner (R < 60) region. Finally, using published density moments from Huscher et al. (2021, https://doi.org/10.1029/2021JA029446), we compared the relative density variations of the Juno moments and the GAMERA simulation to further understand the overall structure and dynamics of the plasmadisc. The agreement between data and simulation supports the existence of such a highly structured plasmadisc.

FEATURES OF EXCITATION AND AZIMUTHAL AND MERIDIONAL PROPAGATION OF LONG-PERIOD Pi3 OSCILLATIONS OF THE GEOMAGNETIC FIELD ON DECEMBER 8, 2017

We study the Pi3 pulsations (with a period T=15–30 min) that were recorded on December 8, 2017 at ground stations in the midnight sector of the magnetosphere at the latitude range of DP2 current system convective electrojets. We have found that Pi3 are especially pronounced in the pre-midnight sector with amplitude of up to 300 nT and duration of up to 2.5 hrs. The pulsation amplitude rapidly decreased with decreasing latitude from F′=72° to F′=63°. The event was recorded during the steady magnetospheric convection. In the southward Bz component of the interplanetary magnetic field, irregular oscillations were detected in the Pi3 frequency range. They correspond to slow magnetosonic waves occurring without noticeable variations in the dynamic pressure Pd. Ground-based geomagnetic observations have shown azimuthal propagation of pulsations with a 0.6–10.6 km/s velocity east and west of the midnight meridian. An analysis of the dynamics of pulsations along the meridian has revealed their propagation to the equator at a velocity 0.75–7.87 km/s. In the projection onto the magnetosphere, the velocities are close in magnitude to the observed propagation velocities of substorm injected electrons. In the dawn-side magnetosphere during ground-observed Pi3 pulsations, compression mode oscillations were recorded.
 We conclude that propagation of geomagnetic field oscillations in this event depends on the dynamics of particle injections under the action of a large-scale electric field of magnetospheric convection, which causes the plasma to move to Earth due to reconnection in the magnetotail. Small-scale oscillations in the magnetosphere were secondary, excited by the solar wind oscillations penetrating into the magnetosphere.

Особенности возбуждения и распространения по азимуту и меридиану длиннопериодных Pi3 колебаний геомагнитного поля 8 декабря 2017 г.

We study the Pi3 pulsations (with a period T=15–30 min) that were recorded on December 8, 2017 at ground stations in the midnight sector of the magnetosphere at the latitude range of DP2 current system convective electrojets. We have found that Pi3 are especially pronounced in the pre-midnight sector with amplitude of up to 300 nT and duration of up to 2.5 hrs. The pulsation amplitude rapidly decreased with decreasing latitude from F′=72° to F′=63°. The event was recorded during the steady magnetospheric convection. In the southward Bz component of the interplanetary magnetic field, irregular oscillations were detected in the Pi3 frequency range. They correspond to slow magnetosonic waves occurring without noticeable variations in the dynamic pressure Pd. Ground-based geomagnetic observations have shown azimuthal propagation of pulsations with a 0.6–10.6 km/s velocity east and west of the midnight meridian. An analysis of the dynamics of pulsations along the meridian has revealed their propagation to the equator at a velocity 0.75–7.87 km/s. In the projection onto the magnetosphere, the velocities are close in magnitude to the observed propagation velocities of substorm injected electrons. In the dawn-side magnetosphere during ground-observed Pi3 pulsations, compression mode oscillations were recorded.
 We conclude that propagation of geomagnetic field oscillations in this event depends on the dynamics of particle injections under the action of a large-scale electric field of magnetospheric convection, which causes the plasma to move to Earth due to reconnection in the magnetotail. Small-scale oscillations in the magnetosphere were secondary, excited by the solar wind oscillations penetrating into the magnetosphere.

Reconnection‐ and Dipolarization‐Driven Auroral Dawn Storms and Injections

AbstractJupiter displays many distinct auroral structures, among which auroral dawn storms and auroral injections are often observed contemporaneously. However, it is unclear if the contemporaneous nature of the observations is a coincidence or part of an underlying physical connection. We show six clear examples from a recent Hubble Space Telescope campaign (GO‐14634) that each display both auroral dawn storms and auroral injection signatures. We found that these conjugate phenomena could exist during intervals of either relatively low or high auroral activity, as evidenced by the varied levels of total auroral power. In situ observations of the magnetosphere by Juno show a strong magnetic reconnection event inside of 45 Jupiter radii (RJ) on the predawn sector, followed by two dipolarization events within the following 2 hr, coincident with the auroral dawn storm and auroral injection event. We therefore suggest that the auroral dawn storm is the manifestation of magnetic reconnection in the dawnside magnetosphere. The dipolarization region is mapped to the auroral injection, strongly suggesting that this was associated with the auroral injection. Since magnetic reconnection and dipolarization are physically connected, we therefore suggest that the often‐conjugate auroral dawn storm and auroral injection events are also physically connected consequences.

Drift‐Dispersed Flux Dropouts of Energetic Electrons Observed in Earth's Middle Magnetosphere by the Magnetospheric Multiscale (MMS) Mission

AbstractEnergetic particle injections are characterized by dispersive or dispersionless increases in observed particle flux. Observations from National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission have revealed transient events displaying injection‐like dispersed reductions in energetic (~30–600 keV) electron flux in the dawnside magnetosphere. Although initially believed to be the result of magnetopause losses, drift tracing of the electrons suggests a source for these drift‐dispersed flux dropouts in the near‐to‐postmidnight magnetotail suggesting that they are likely related to similar signatures previously observed at geosynchronous orbit. We suggest that the dozen examples presented are signatures of “flux‐reduced injections” resulting from earthward injection in the presence of a negative phase space density radial gradient as supported by observed phase space density versus L‐shell profiles. These events also display varying pitch angle responses inconsistent with a singular loss mechanism, leading to the suggestion that they result from preconditioning of the magnetotail source region prior to the injection.

The Rapid Responses of Magnetosonic Waves to the Compression and Expansion of Earth's Magnetosphere

AbstractUnder different solar wind dynamic pressures, we observed the magnetosonic (MS) wave amplification and attenuation associated with the compression and expansion of the Earth's magnetosphere. By analyzing the wave and particle variations recorded by the twin Van Allen Probes, we found that the magnetospheric compression or expansion can alter the keV proton phase space density distribution in velocity space and thus affects the MS wave intensity in upper band (f > 50 Hz) in the dawnside magnetosphere (magnetic local time ~ 4.0–7.9 and L ~ 5.7–3.0). During the magnetospheric compression period, the reduction of the 0.1 to 2 keV protons and the enhancement of the 3 to 7 keV protons form a positive phase space density gradient in their velocity space (i.e., the proton ring distribution), and meanwhile, the upper band MS waves are significantly amplified in the proton ring distribution region. During the subsequent magnetospheric expansion period, the enhancement of the 0.1 to 2 keV protons and the reduction of the protons above 3 keV produce a negative phase space density gradient in their velocity space, and meanwhile, the upper band MS waves are rapidly damped. These observations demonstrate that the intensity of the upper band MS waves in the dawnside magnetosphere is highly variable during the change in solar wind pressure.

Global Structure and Sodium Ion Dynamics in Mercury's Magnetosphere With the Offset Dipole

AbstractWe conducted global magnetohydrodynamics (MHD) simulation of Mercury's magnetosphere with the dipole offset, which was revealed by MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) observations, in order to investigate its global structure under northward interplanetary magnetic field conditions. Sodium ion dynamics originating from the Mercury's exosphere is also investigated based on statistical trajectory tracing in the electric and magnetic fields obtained from the MHD simulations. The results reveal a north‐south asymmetry characterized by open field lines around the southern polar region and northward deflection of the plasma sheet in the far tail. The asymmetry of magnetic field structure near the planet drastically affects trajectories of sodium ion and thus their pressure distributions and precipitation pattern onto the planet. Weaker magnetic field strength in the southern hemisphere than in the north increases ion loss by precipitation onto the planetary surface in the southern hemisphere. The “sodium ring,” which is formed by high‐energy sodium ions drifting around the planet, is also found in the vicinity of the planet. The sodium ring is almost circular under nominal solar wind conditions. The ring becomes partial under high solar wind density, because dayside magnetosphere is so compressed that there is no space for the sodium ions to drift around. In both cases, the sodium ring is formed by sodium ions that are picked up, accelerated in the magnetosheath just outside the magnetopause, and reentered into the magnetosphere due to combined effects of finite Larmor radius and convection electric field in the dawnside magnetosphere.

A statistical survey of ultralow-frequency wave power and polarization in the Hermean magnetosphere.

We present a statistical survey of ultralow‐frequency wave activity within the Hermean magnetosphere using the entire MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data set. This study is focused upon wave activity with frequencies <0.5 Hz, typically below local ion gyrofrequencies, in order to determine if field line resonances similar to those observed in the terrestrial magnetosphere may be present. Wave activity is mapped to the magnetic equatorial plane of the magnetosphere and to magnetic latitude and local times on Mercury using the KT14 magnetic field model. Wave power mapped to the planetary surface indicates the average location of the polar cap boundary. Compressional wave power is dominant throughout most of the magnetosphere, while azimuthal wave power close to the dayside magnetopause provides evidence that interactions between the magnetosheath and the magnetopause such as the Kelvin‐Helmholtz instability may be driving wave activity. Further evidence of this is found in the average wave polarization: left‐handed polarized waves dominate the dawnside magnetosphere, while right‐handed polarized waves dominate the duskside. A possible field line resonance event is also presented, where a time‐of‐flight calculation is used to provide an estimated local plasma mass density of ∼240 amu cm−3.

Quasi-periodic injections of relativistic electrons in Saturn’s outer magnetosphere

Quasi-periodic, short-period injections of relativistic electrons have been observed in both Jupiter’s and Saturn’s magnetospheres, but understanding their origin or significance has been challenging, primarily due to the limited number of in-situ observations of such events by past flyby missions. Here we present the first survey of such injections in an outer planetary magnetosphere using almost nine years of energetic charged particle and magnetic field measurements at Saturn. We focus on events with a characteristic period of about 60–70min (QP60, where QP stands for quasi-periodic). We find that the majority of QP60, which are very common in the outer magnetosphere, map outside Titan’s orbit. QP60 are also observed over a very wide range of local times and latitudes. A local time asymmetry in their distribution is the most striking feature, with QP60 at dusk being between 5 and 25 times more frequent than at dawn. Field-line tracing and pitch angle distributions suggest that most events at dusk reside on closed field lines. They are distributed either near the magnetopause, or, in the case of the post-dusk (or pre-midnight) sector, up to about 30RS inside it, along an area extending parallel to the dawn–dusk direction. QP60 at dawn map either on open field lines and/or near the magnetopause. Both the asymmetries and varying mapping characteristics as a function of local time indicate that generation of QP60 cannot be assigned to a single process. The locations of QP60 seem to trace sites that reconnection is expected to take place. In that respect, the subset of events observed post-dusk and deep inside the magnetopause may be directly or indirectly linked to the Vasyliunas reconnection cycle, while magnetopause reconnection/Kelvin–Helmholtz (KH) instability could be invoked to explain all other events at the duskside. Using similar arguments, injections at the dawnside magnetosphere may result from solar-wind induced storms and/or magnetopause reconnection/KH-instability. Still, we cannot exclude that the apparent collocation of QP60 with expected reconnection sites is coincidental. given also the large uncertainties in field line tracing with the available magnetic field models. The intensity of the QP60 spectrum is strong enough such that if transport processes allow, these injections can be a very important source of energetic electrons for the inner saturnian magnetosphere or the heliosphere. We also observe that electrons in a QP60 can be accelerated at least up to 6MeV and that the distribution of QP60 appears to trace well the aurora’s local time structure, an observation that may have implications about high-latitude electron acceleration and the connection of these events to auroral dynamics. Despite these new findings, it is still unclear what determines the rather well-defined 60 to 70-min period of the electron bursts and how electrons can rapidly reach several MeV.

THEMIS observations of compressional poloidal pulsations in the dawnside magnetosphere: A case study

We present results from a case study of compressional poloidal pulsations with periods ranging from 5 to 12 min in the dawn magnetosphere from 1630 to 2000 UT on 27 February 2011. However, we employ THEMIS (Time History of Events and Macroscale Interactions during Substorms) magnetic field, plasma, and energetic particle observations to determine azimuthal wavelengths (7000–27,000 km) and velocities (9–28 km/s) consistent with previous work. We then apply a new method invoking the north to south motion of the THEMIS spacecraft and the fact that the equatorial node exhibited north‐south oscillations to derive the meridional structure of the wave in great detail. We determined that the amplitude of the meridional motion of the equatorial node was ~0.8 RE, and that the meridional wave length exceeded 3 RE.

Frequency doubling and field‐aligned ion streaming in a long‐period poloidal pulsation

We reexamine a long‐period (10–18 min) poloidal pulsation observed by THEMIS‐A in the outer dawn magnetosphere from 10:00 to 12:30 UT on 7 November 2007. The interval was originally reported by Korotova et al. (2009). Although the nonlinear compressional plasma and magnetic field perturbations observed by THEMIS‐A during this interval agree well with model predictions for the linear perturbations associated with antisymmetric waves generated by the ballooning‐mirror mode instability, the phase relationships between these perturbations indicates a complex frequency rather than the purely imaginary frequency that theory predicts for the outer dawnside magnetosphere. Variations in the radial plasma velocity confirm that a phase‐locked north/south oscillation in the equatorial line of nodes associated with the ballooning‐mirror mode waves doubles the frequency of the compressional component of the magnetic field during these pulsations. The same velocity and magnetic field perturbations exclude explanations for the frequency doubling in terms of spatial gradients sweeping back and forth across the spacecraft or drift‐bounce resonances. Azimuthal electric fields associated with the pulsations generate field‐aligned anisotropies in the pitch angle distributions that become more prominent with increasing ion energy due to the presence of drift‐shell splitting and radial flux gradients that steepen with increasing energy. Although there was no evidence for drift‐bounce interactions during this event, the role of such events in ion energization in other events and at other locations remains to be evaluated.

THEMIS observations of compressional pulsations in the dawn-side magnetosphere: a case study

Abstract. We present THEMIS-A low- and high-energy plasma, magnetic field, and energetic particle observations of long period (11–36 min) irregular compressional pulsations in the dawnside magnetosphere from 08:00 to 12:24 UT on 7 November 2007. We demonstrate that the pulsations maintain thermal and magnetic pressure balance, then employ finite gyroradius techniques to determine wave properties from the gyrophase distributions of 5–10 keV ions. The waves generally move sunward at velocities ~10 km s−1 with the background plasma convection flow. Wavelengths range from 6700 to 23 300 km, corresponding to azimuthal wavenumbers m from 18 to 76. Wave periods decrease with increasing radial distance. Having determined the parameters describing the waves, we consider three previously proposed explanations: generation by substorm injection, generation by bounce or drift-bounce instabilities, and generation by the drift-mirror instability. The interval was quiet geomagnetically, arguing against any relationship to substorm injections. We found that ions with low energies of 69–628 eV or high energies of 28–615 keV would have been required to account for drift-bounce resonance during this interval, but inspection reveals ion fluxes at these energies near background levels during the time period considered. On the other hand, the criteria for the drift mirror instability are marginally satisfied. As predicted for the drift mirror instability, particle distributions peak more sharply near 90° pitch angles during magnetic field strength enhancements than during strength depressions. At this point we therefore interpret the compressional pulsations observed by THEMIS A in terms of the drift mirror instability.

Surveys on magnetospheric plasmas based on the Double Star Project (DSP) exploration

The equatorial and polar satellites of the Double Star Project (DSP) were launched successfully on December 29, 2003 and July 25, 2004, respectively, and both of them are operating smoothly. The DSP provides a good opportunity for investigating the structure of the magnetosphere. Based on the DSP data collected during 2004, we have surveyed the distribution of the magnetic fields and plasmas in the magnetosphere. It is found that: (1) Near the Earth’s equatorial plane within geocentric distances of less than 7 RE, the Earth’s magnetic field is dipolar. In the vicinity of the magnetopause, the magnetic field is enhanced by a factor of about 1.5, and on the nightside, the magnetic field can vary significantly from the Earth’s dipole field, likely caused by the presence of the near-Earth tail current sheet. (2) In the day-side magnetosheath, the electron and ion densities are usually both in the range of 10–30 cm−3; the ion and electron temperatures are usually about 200 and 50 eV, respectively. The flow pattern is usually smooth, with a low velocity in the subsolar region and with significantly higher velocities in the dawn and dusk flanks. (3) In the region between the magnetopause and plasmasphere the density is low, approximately 0.5–5 cm−3, and the temperature is high, about 1–10 keV for ions and 0.1–5 keV for electrons. The ion temperature has an apparent anisotropy, with the ratio of the perpendicular and parallel temperatures being about 1.0–1.3 for the night-and dusk-side magnetosphere and about 1.3–2.0 for the day-and dawn-side magnetosphere. There is an evident sunward convection of about 50 km/s in the magnetosphere. On the dawn side, the flow becomes somewhat turbulent, and in the vicinity of the night-noon meridian plane, the convection is rather slow. (4) The high-energy electrons with energies higher than 2 MeV are mainly located in the regions with 3 < L < 4.5; the size of the high-energy electrons area varies with time, it may expand and shrink occasionally according to different solar wind conditions and magnetic activities.

Electron structure of the magnetopause boundary layer: Cluster/Double Star observations

We present a comparison of two events, monitored by the Double Star and Cluster spacecraft at separate locations on the dayside magnetopause, which exhibit distinct properties at high and low latitudes in the magnetopause boundary layer during the occurrence of low‐latitude reconnection. On 6 April 2004, the four Cluster and TC‐1 spacecraft were on near‐coincident, outbound transits of the dawnside magnetosphere at north and south midlatitudes, respectively. The observations show a series of oppositely directed flux transfer events (FTEs), fed by a low‐latitude reconnection line located between the spacecraft. Although both spacecraft locations were nearly equidistant from the active reconnection region, the associated magnetopause boundary layer was maintained at TC‐1 but not at Cluster. We suggest an asymmetric north and south extent of the LLBL so as to be more extensive at TC‐1, where the local magnetic shear across the magnetopause is small. On 4 January 2005, the Cluster and TC‐1 spacecraft all repeatedly traverse the northern, duskside magnetopause almost simultaneously, before and after a strong reversal in the IMF from northward to southward during a period of turbulent solar wind. Open flux tubes are observed within minutes of the southward turning, arising from the sudden formation of a nearby subsolar reconnection line. Before the IMF change, a complex and energized boundary layer, largely absent at the lower latitudes of TC‐1, and containing an energetic (&gt;40 keV) electron population of locally trapped and field‐aligned distributions, is present at the high‐latitude Cluster locations. Following reconnection onset after the IMF reversal, the boundary layer is seen to extend to TC‐1, and the electron distribution, which depends on position through the boundary layer, develops as an energetic, field‐aligned (bistreaming) distribution. The analysis is utilizes an extended electron distribution for energies ranging from a few to 400 keV and by reordering the transition through the magnetopause to the electron distribution.

Equator‐S observations of drift mirror mode waves in the dawnside magnetosphere

The mirror mode is a plasma instability that is typically excited in high‐beta plasmas where there is significant pressure anisotropy and is most commonly observed in the magnetosheath. However, it is possible under sufficiently anisotropic conditions to generate the mirror instability inside the magnetosphere, though as yet there are few examples. We present an extended interval of ∼7 hours of mirror mode activity on 19 March 1998 when the Equator‐S spacecraft was traversing the dawnside magnetosphere above the equatorial plane at radial distances of up to L ∼ 11 and encountered quasi‐monochromatic ∼2‐min oscillations in magnetic field and ion number density, temperature, and velocity. The magnetic field strength and number densities were in antiphase, and the plasma and magnetic field pressure perturbations were also in antiphase while the observed total pressure remained constant. This is consistent with excitation via a mirror instability. We are able to discern that the mirror activity must be confined to approximately within ±20° of the equatorial plane through a conjunction with the Geotail spacecraft. We find that the condition for mirror mode waves to grow is strongly met throughout the interval (Hasegawa, 1969). We believe that this is an excellent high temporal resolution example of the mirror instability exciting ULF waves inside the Earth's magnetosphere. Given the coexistence of toroidal oscillations at almost the same frequency, we suggest coupling between the mirror mode and local standing Alfvén waves. Our observations hence add to the understanding of how energy can be transferred from hot plasma into ULF waves in the magnetosphere before being dissipated in the ionosphere.