Bow Shock Acceleration Research Articles

Case Study of Solar Wind Suprathermal Electron Acceleration at the Earth’s Bow Shock

Abstract We present a case study of the in situ acceleration of solar wind suprathermal electrons at the two quasi-perpendicular-bow-shock crossings on 2015 November 4, combining the Wind 3D Plasma and Energetic Particle measurements of ambient solar wind suprathermal electrons and Magnetospheric Multiscale mission measurements of shocked suprathermal electrons. In both cases, the omnidirectional differential fluxes of shocked suprathermal electrons in the downstream exhibit a double-power-law energy spectrum with a spectral index of ∼3 at energies below a downward break ε brk near 40 keV and index of ∼6 at energies above, different from the unshocked suprathermal electrons observed in the ambient solar wind. At energies below (above) ε brk, the observed electron flux ratio between the downstream and ambient solar wind, J D /J A , peaks near 90° PA (becomes roughly isotropic). Electrons at ε brk have an average electron gyrodiameter (across bow shock) comparable to the shock thickness. These suggest that the bow-shock acceleration of suprathermal electrons is likely dominated by the shock drift acceleration mechanism. For electrons at energies below (above) ε brk, their estimated drift time appears to be roughly energy independent (decrease with energy), leading to the formation of a double-power-law spectrum substantially steepening at a break that’s determined by the shock thickness.

MeV ion event observed at 0950 UT on 4 May 1998 at a quasi-perpendicular bow shock region: New observations and an alternative interpretation on its origin

[1] The region near the Earth's bow shock provides a chance to test and compare various charged particle acceleration models (magnetospheric versus bow shock acceleration processes). This kind of research attracted the interest of a large part of the space community in the last four decades. During the last decade, some ion events with spectrum extending to high energies (greater than ∼1.3 MeV) observed by Polar on 4 May 1998 provided the opportunity for a new debate. These ion events were analyzed in a large number of papers and were discussed in terms of acceleration processes in the cusp of the magnetosphere or at a quasi-parallel region of the bow shock. In this paper, we reexamine in detail one of the ion events observed during the period of interest, around ∼0950 UT, and we provide new crucial information and interpretation by focusing on data obtained very close to the shock (not examined so far). At ∼0950 UT, Polar crossed a quasi-perpendicular bow shock under special solar wind speed/bow shock conditions, which favor the efficient acceleration of the solar ambient energetic population via the shock drift acceleration (SDA) mechanism. We analyze in detail observations upstream and downstream from the bow shock crossing of 0945 UT, for instance, flux-time profiles, energy spectra, ion composition, and pitch angle distributions, and we find them, as expected, in excellent agreement with the predictions of the SDA theory.

Injection of energetic protons during solar eruption on 1999 May 9: Effect of flare and coronal mass ejection

A solar energetic particle (SEP) event was observed on 1999 May 9 by the Energetic and Relativistic Nuclei and Electron instrument (ERNE) onboard the Solar and Heliospheric Observatory (SOHO) spacecraft in association with a coronal mass ejection (CME) and X-ray flare at the western limb. Near flare onset the active region coronal loop structure was seen to erupt and simultaneous blue and red shift velocities of the hot plasma were recorded by the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument onboard SOHO. We observe for the first time three periods of the SEP injection in a single event: (i) the first, extremely-hard spectrum injection triggered by the passage of the flare initiated coronal (shock) wave; (ii) a moderately-hard spectrum phase starting about half a hour later, proceeding and ceasing concurrently with metric continuum radio burst; (iii) a prolonged soft spectrum injection dominating in the late phase of the event, after about 1.5 h from the first proton production. The CME bow shock acceleration provides a straightforward explanation of the final spectral redressing, whereas the first acceleration seems triggered by the flare. These observations lead us to conclude that the 1999 May 9 SEP event was caused by a combination of coronal and interplanetary acceleration processes contributing with varying importance at different stages of the solar eruption associated with both flare and CME. Comparison with other events suggests that it is a common property of mixed SEP events.

The Origin of Solar Energetic Particle Events: Coronal Acceleration versus Shock Wave Acceleration

We review evidence that led to the view that acceleration at shock waves driven by coronal mass ejections (CMEs) is responsible for large particle events detected at 1 AU. It appears that even if the CME bow shock acceleration is a possible model for the origin of rather low energy ions, it faces difficulties on account of the production of ions far above 1 MeV: (i) although shock waves have been demonstrated to accelerate ions to energies of some MeV nucl−1 in the interplanetary medium, their ability to achieve relativistic energies in the solar environment is unproven; (ii) SEP events producing particle enhancements at energies ≥100 MeV are also accompanied by flares; those accompanied only by fast CMEs have no proton signatures above 50 MeV. We emphasize detailed studies of individual high energy particle events which provide strong evidence that time-extended particle acceleration which occurs in the corona after the impulsive flare contributes to particle fluxes in space. It appears thus that the CME bow shock scenario has been overvalued and that long lasting coronal energy release processes have to be taken into account when searching for the origin of high energy SEP events.

Medium energy proton fluxes outside the magnetopause: INTERBALL-1 data

The extensive data set of the DOK2 energetic proton measurements indicates a decrease of flux at 22 – 29 keV based on the distance from the magnetopause within the magnetosheath. In the region upstream from the bow shock the signatures expected for the preferential acceleration of protons at quasiparallel shocks are found in the data. In addition to the proton leakage from the magnetosphere observed in several experiments, the observations confirm the importance of bow shock acceleration for the population of medium energy protons in the near upstream region.

Correlation of cusp MeV helium with turbulent ULF power spectra and its implications

A new magnetospheric phenomenon called a cusp energetic particle (CEP) event was observed by the POLAR spacecraft. CEP events were detected in the dayside polar cusp near the apogee of POLAR and could last for hours, in which the measured helium ions had energies up to 8 MeV [Chen et al., 1997a, 1998]. A fundamental question is where do the cusp MeV ions come from? To answer this question, we have compared the ion flux in the CEP events with that in both the upstream and the downstream from the bow shock and found that bow shock acceleration cannot explain the measured ion flux in the CEP events. We have further determined the power spectra of the local magnetic field turbulence calculated over the September 18, 1996 CEP event periods for fluctuations in the ultra‐low frequency (ULF) ranges, corresponding to periods of about 0.33–500 seconds. It is found that the integrated power of the turbulent spectra over the ULF ranges is correlated with the intensity of the MeV helium flux. These new results provides the first direct observation evidence that the high‐altitude dayside cusp is a new acceleration region of the magnetosphere.

Observation and analysis of selected IMP 8 and WIND bow shock crossings in late 1994

Nine bow shock crossings from late 1994 are examined using data from the IMP 8 spacecraft as input to the Rankine‐Hugoniot (RH) conservation equations. In addition, two crossings from the same time period are examined using WIND spacecraft data. A nonlinear chi‐square minimization method is used to obtain the best fit bow shock (BS) normal and speed for each crossing. The shock normals and speeds are also calculated neglecting temperature, as well as using only the magnetic coplanarity and velocity coplanarity techniques. All of the methods generally give mutually consistent results. The orientations of the shock normals are consistent with the nominal bow shock shape. The shocks were expected to move according to a simple “breathing model in which the bow shock moves inward and outward from Earth according to changes in external solar wind conditions. However, the results show outward motion of the bow shock for all but one of all the crossings considered. Moreover, the direction of motion of the BS depends on the exact choice of the shock normal so that a slightly different shock normal can give a different sense of motion of the BS. We conclude that the RH equations that we use may be missing some terms which would provide an accurate description of the balance on both sides of the BS. In particular, the BS acceleration at its flanks is not taken into account by these equations.

Prognoz 10 energetic particle data: Leakage from the magnetosphere versus bow shock acceleration

We present a case study and a statistical survey of Prognoz 10 energetic particle and magnetic field observations from the solar wind to the magnetosphere. The results provide evidence for both Fermi acceleration and leakage from the magnetosphere. Energetic ion (E > 15 keV) flux levels upstream of the bow shock and in the outer magnetosphere generally exceed those in the magnetosheath. Energetic electrons rarely accompany ion fluxes in the outer magnetosheath and upstream of the bow shock, but commonly accompany those in the inner magnetosheath. Upstream flux levels increase as the interplanetary magnetic field (IMF) rotates toward the bow shock normal, consistent with Fermi acceleration. Although the north/south magnetosheath magnetic field orientation has no effect upon energetic particle flux levels in the outer magnetosheath, flux levels in the inner magnetosheath increase during periods of southward IMF. We interpret the results as indicating that Fermi acceleration supplies most of the ions observed in the outer dayside magnetosheath but that leakage from the magnetosphere supplies most of the ions observed in the inner magnetosheath.

Medium energy particle perspective from magnetopause to upstream region: Prognoz-10 data

We present typical examplesof Prognoz-10 ion (E>15 keV) and electron (E> keV) flux observations as the spacecraft crosses the low and mid-latitude magnetosheath and the foreshock. Both leakage from magnetosphere and bow shock acceleration contribute to magnetosheath populations. A statistical survey using 6 months of measurements reveals that the occurence of high ion and electron fluxes depends upon K p. For the same K p, the probability of high ion occurence is much higher in the magnetosheath than in the upstream region. In the upstream region, the occurence of high ion fluxes depends upon the distance to the bow shock and on the angle Θ Bn.

Correlated observations of energetic particles upstream of the bow shock, in the magnetosheath, and in the magnetosphere

Simultaneous measurements of energetic protons and alpha particles by the ISEE 1 and AMPTE/IRM spacecraft in the region upstream of the Earth's bow shock are reported during the period 0700–0900 UT on October 19, 1984. IRM was positioned at about 0900 local time close to the bow shock, whereas ISEE 1 was to the duskside of the bow shock and further upstream. The magnetic field was on average in the Parker spiral direction, so that IRM was connected to the parallel bow shock whereas ISEE 1 was connected for most of the time to the perpendicular bow shock. IRM observes a diffuse distribution over the whole two hour period. In the spacecraft frame an anisotropy exists with more particles streaming toward the bow shock. ISEE 1 observes occasional bursts of energetic ions which exhibit a beam like distribution along the magnetic field away from the bow shock. The occurrence of these beams is in most cases correlated with a change of the longitudinal interplanetary magnetic field direction, such that a field line from the position of the spacecraft meets the shock with a smaller angle between magnetic field and shock normal. The differential intensity at IRM is about one order of magnitude higher than at ISEE 1. Simultaneously measured spectra by AMPTE/CCE within the magnetosphere close to the magnetopause are considerably harder than the spectra at both upstream locations. A comparison of a CCE spectrum in the magnetosheath with an upstream IRM spectrum shows that the proton intensity observed just outside of the magnetopause between 15 and 80 keV is lower than the upstream spectrum. In the magnetosheath the protons stream away from the bow shock toward the magnetopause and the anisotropy increases with energy. Predictions of the magnetospheric leakage model and of the bow shock acceleration model are compared, and it is shown that the observations are consistent with bow shock acceleration, whereas there are extreme difficulties with an explanation in terms of magnetospheric leakage.

Observations of significant magnetosheath antisolar energy flow

Evidence for significant but sporadic magnetosheath energy flows in the antisolar direction is presented, using observations of 50 to 220‐keV ions made by the NOAA/Applied Physics Laboratory energetic particle experiment on the Imp 7 satellite. The observations have been made in the dawn and dusk magnetosheath at distances of ∼35 RE from the earth and cover a wide range of ZSM. Direct observations of 50‐ to 220‐keV ions yield an energy flow for the case studied of 2(10)−3−2(10)−2 erg/cm² s. By using a somewhat arbitrary but reasonable magnetosheath area based on 1 year of synoptic survey of such flows a total magnetosheath ≥50‐keV ion energy flow of ≳1018 ergs/s is obtained. Reasonable spectral extrapolations yield somewhat larger total flows. These values indicate that ions carry at least an equal portion of the energetic particle magnetosheath energy flow as the extrapolated >1‐keV electron flow obtained by Baker and Stone (1977). Possible sources of this energy flow include dissipation processes due to a magnetopause electric field, leakage of magnetospheric particles through the magnetopause into the magnetosheath, bow shock acceleration, and magnetosheath acceleration.