Suprathermal Heavy Ions Research Articles

On the response of Martian Ionosphere to the Passage of a Corotating Interaction Region: MAVEN Observations

AbstractThe response of Martian ionosphere to the passage of corotating interaction region (CIR) of June 2015 is studied using observations from several instruments aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. An intense CIR arrived at Mars on 22 June 2015, during which the upstream solar wind and interplanetary magnetic field conditions were monitored by the Solar Wind Ion Analyzer (SWIA), Solar Wind Electron Analyzer (SWEA), Solar Energetic Particle (SEP), and Magnetometer (MAG) instruments aboard MAVEN. The CIR event was characterized by enhancements in solar wind density, velocity, and dynamic pressure, and increased and fluctuating interplanetary magnetic field and was associated with enhanced fluxes of Solar Energetic Particles. The Langmuir Probe and Waves (LPW) instrument onboard MAVEN provided the ionospheric observations such as electron density and electron temperature during this period. The dayside ionosphere is significantly compressed only near the peak of solar wind dynamic pressure enhancement (∼14 nPa). In contrast, on the nightside, the electron density remains depleted for a longer period of time. The electron temperatures are also enhanced during the period of electron depletion on the nightside. The Suprathermal and Thermal Ion Composition (STATIC) measurements show enhanced fluxes of suprathermal heavy ions in the Martian exosphere during CIR period, and evidences for enhanced tailward flow of these pickup ions. The analysis suggests that the nightside ionosphere is primarily controlled by the precipitating Solar Energetic Particles and pickup ions transported across the Martian terminator and depletes significantly when the heavy ion flux in the exosphere enhances.

Spectral Properties and Abundances of Suprathermal Heavy Ions in Compression Regions near 1 au

Abstract We have analyzed spectral properties and abundances of ∼0.02–3.0 MeV nucleon−1 suprathermal (ST) H–Fe ions in 41 stream interaction regions (SIRs) near 1 au observed by Wind and ACE spacecraft from 1995 January through 2008 December. We find that, (i) the event-averaged spectral index is γ ∼ 2.44, with a standard deviation (σ) of 0.67, (ii) γ’s are poorly correlated with the magnetic compression ratios, and 17% of the events group around γ ∼ 1.5, (iii) γ’s for both O and Fe at ∼0.02–0.09 MeV nucleon−1 and 0.09–0.3 MeV nucleon−1 are correlated, but do not exhibit any systematic steepening or flattening as a function of energy, (iv) the ST heavy ion abundance ratios remain constant with increasing energy, implying that the spectral rollovers, defined by the e-folding energy E 0, are independent of the ion’s mass per charge (M/Q), and (v) SIR ST abundances are similar to the corresponding solar wind values, and do not exhibit any systematic behavior when plotted versus the ion’s M/Q or first ionization potential. The above results pose challenges for (1) particle acceleration models that invoke either a corotating interaction region or SIR shocks between ∼3 and 5 au, (2) particle transport models that predict M/Q-dependent spectral rollovers due to interplanetary turbulence effects, and (3) the notion that SIR ST ions originate directly from the bulk solar wind. Instead, we suggest that the SIR ST ions are accelerated out of a pool of material that includes particles accelerated in solar energetic particle events and processed or heated solar wind ions.

Source Population and Acceleration Location of Suprathermal Heavy Ions in Corotating Interaction Regions

Abstract We have analyzed the ∼20–320 keV nucleon−1 suprathermal (ST) heavy ion abundances in 41 corotating interaction regions (CIRs) observed by the Wind spacecraft from 1995 January to 2008 December. Our results are: (1) the CIR Fe/CNO and NeS/CNO ratios vary with the sunspot number, with values being closer to average solar energetic particle event values during solar maxima and lower than nominal solar wind values during solar minima. The physical mechanism responsible for the depleted abundances during solar minimum remains an open question. (2) The Fe/CNO increases with energy in the 6 events that occurred during solar maximum, while no such trends are observed for the 35 events during solar minimum. (3) The Fe/CNO shows no correlation with the average solar wind speed. (4) The Fe/CNO is well correlated with the corresponding upstream ∼20–320 keV nucleon−1 Fe/CNO and not with the solar wind Fe/O measured by ACE in 31 events. Using the correlations between the upstream ∼20–40 keV nucleon−1 Fe/CNO and the ∼20–320 keV nucleon−1 Fe/CNO in CIRs, we estimate that, on average, the ST particles traveled ∼2 au along the nominal Parker spiral field line, which corresponds to upper limits for the radial distance of the source or acceleration location of ∼1 au beyond Earth orbit. Our results are consistent with those obtained from recent surveys, and confirm that CIR ST heavy ions are accelerated more locally, and are at odds with the traditional viewpoint that CIR ions seen at 1 au are bulk solar wind ions accelerated between 3 and 5 au.

EVIDENCE FOR LOCAL ACCELERATION OF SUPRATHERMAL HEAVY ION OBSERVATIONS DURING INTERPLANETARY CORONAL MASS EJECTIONS

Suprathermal particles are an important seed population for a variety of energetic particles found throughout the heliosphere, but their origin is in debate. We present, for the first time, high-cadence observations of suprathermal heavy ions during interplanetary coronal mass ejections (ICMEs), from the Suprathermal Ion Composition Spectrometer on board the Wind spacecraft, and investigate their ionic composition and compare it to the bulk solar wind plasma composition, observed from the Solar Wind Ion Composition Spectrometer on board the Advanced Composition Explorer. We find that the composition of the suprathermal plasma is related to the local bulk solar wind plasma and not to the plasma upstream of the ICME. This implies that the suprathermal plasma is accelerated from the local bulk solar wind plasma and not the upstream solar wind plasma.

Abundances of Suprathermal Heavy Ions in CIRs During the Minimum of Solar Cycle 23

In this paper we examine the elemental composition of the 0.1 – 1 MeV nucleon−1 interplanetary heavy ions from H to Fe in corotating interaction regions (CIRs) measured by the SIT (Suprathermal Ion Telescope) instrument. We use observations taken on board the STEREO spacecraft from January 2007 through December 2010, which included the unusually long solar minimum following Solar Cycle 23. During this period instruments on STEREO observed more than 50 CIR events making it possible to investigate CIR ion abundances during solar minimum conditions with unprecedented high statistics. The observations reveal annual variations of relative ion abundances in the CIRs during the 2007 – 2008 period as indicated by the He/H, He/O and Fe/O elemental ratios. We discuss possible causes of the variability in terms of the helium focusing cone passage and heliolatitude dependence. The year 2009 was very quiet in CIR-event activity. In 2010 the elemental composition in CIRs were influenced by sporadic solar energetic particle (SEP) events. The 2010 He/H and He/O abundance ratios in CIRs show large event-to-event variations with values resembling the SEP-like composition. This finding suggests that the suprathermal SEPs could be the source population for CIR acceleration.

COMPOSITION AND SPECTRAL PROPERTIES OF THE 1 AU QUIET-TIME SUPRATHERMAL ION POPULATION DURING SOLAR CYCLE 23

We have surveyed the spectral and compositional properties of suprathermal heavy ions during quiet times from 1995 January 1 to 2007 December 31 using Wind/Energetic Particles: Anisotropy, Composition, and Transport/SupraThermal-through-Energetic Particle Telescope and Advanced Composition Explorer/Ultra-low Energy Isotope Spectrometer at energies between 0.04 and 2.56 MeV nucleon–1. We find the following. (1) Quiet-time Fe/O and C/O abundances are correlated with solar cycle activity, reflecting corresponding values measured in solar energetic particle and interplanetary (IP) shock events during solar maximum, and those measured in the solar wind and corotating interaction regions (CIRs) during solar minimum conditions. (2) The 3He/4He ratio lies in the 3%-8% range during the quiet times of 1998-2004 with finite 3He detected on ~27.4% of the days. This ratio drops to 0.3%-1.2% during 2005-2007 and finite 3He is detected on ~5% of the days. (3) All heavy-ion species exhibit suprathermal tails between 0.04 and 0.32 MeV nucleon–1 with spectral indices ranging from ~1.27 to 2.29. These tails sometimes extend above ~2 MeV nucleon–1 with Fe spectra rolling over at lower energies than those of CNO. (4) The suprathermal tail spectral indices of heavier species (i.e., Fe) are harder than those of the lighter ones (i.e., CNO). These indices do not exhibit a clear solar cycle dependence and for ~50% of the time, they deviate significantly from the 1.5 value. These compositional observations provide evidence that even during the quietest times in IP space, the suprathermal population (3He and C-through-Fe) consists of ions from different sources whose relative contributions vary with solar activity. The heavy-ion energy spectra exhibit suprathermal tails with variable spectral indices that do not exhibit the spectral index of 1.5 predicted by some recent models.

The relationship between suprathermal heavy ion outflow and auroral electron energy deposition: Polar/Ultraviolet Imager and Fast Auroral Snapshot/Time‐of‐Flight Energy Angle Mass Spectrometer observations

Ionospheric ions are energized to suprathermal energies (10–1000 eV) in the auroral zone. This produces a much larger quantity of escaping O+ ions than would otherwise occur, given typical ionospheric energies. Until recently, only limited work had been done relating ion upflow characteristics to nearby, contemporaneous auroral forms. We present our results comparing the characteristics of the suprathermal outflowing O+ ions, as measured by the Time‐of‐Flight Energy Angle Mass Spectrometer instrument on the Fast Auroral Snapshot (FAST) spacecraft, to the auroral forms seen at the foot point of the associated field line, as observed by the Ultraviolet Imager (UVI) on Polar. We present data from FAST nightside auroral zone passes between January 25 and February 11, 1997. During this interval, FAST made ∼100 auroral zone passes in the Northern Hemisphere where the aurora was simultaneously imaged by the UVI. Close examination of 50 such passes shows that the regions where suprathermal O+ outflow occurs closely follow the local aurora regardless of how convoluted the auroral forms may be. Taken together, these data show that the flux of escaping O+ ions increases by over a factor of 100 as the auroral intensity in the 1600–1800 Å band increases from 0 to 4 kR. Also, the delay between auroral intensification and saturation O+ flux reaching 3000‐ to 4000‐km altitude is ∼5–10 min.