Investigation of Small‐Scale Electron Density Irregularities Observed by the Arase and Van Allen Probes Satellites Inside and Outside the Plasmasphere

Abstract

The plasmasphere is the innermost region of the Earth's magnetosphere, which is filled with cold (low energy ∼1-10 electron volt (eV) and dense (10-10 4 cm −3)) plasma of ionospheric origin trapped to the Earth's magnetic field, forming a thermal plasma cloud encircling the Earth (Darrouzet et al., 2009b; Lemaire et al., 1998). Outside the plasmasphere, the plasma characteristics change abruptly to tenuous (1 cm −3), hot (high energy ∼100 eV) plasma. The boundary that separates this outer region of the highly energized, low-density plasma from the plasmasphere, is called plasmapause. The plasma motions in the inner magnetosphere are dominated by large scale electric fields. The corotation electric field enforces the corotation of cold plasma near the Earth, and the magnetospheric convection electric field (generated by the interaction between the solar wind and the magnetosphere) causes the loss of cold plasma outside the plasmasphere. The convection electric field, being a key factor for the formation of the plasmapause (Pierrard et al., 2008), the location and sharpness of the plasmapause vary with

Magnetospheric Mass Density as Determined by ULF Wave Analysis

“…Other techniques include EUV imaging of the plasmasphere (Sandel et al, 2003), energetic FIGURE 11 | Magnetoseismic analysis of toroidal waves detected by the Arase spacecraft in the midnight sector (after Takahashi et al, 2018). (A) Magnetic field components in a magnetic-field-aligned coordinate system based on the T89c magnetic field model (Tsyganenko, 1989) neutral atom remote (ENA) sensing of energetic ions (Roelof et al, 1985;Brandt et al, 2005), estimation of n e using whistler waves (Carpenter and Smith, 1964;Park, 1974), spacecraft potential (Pedersen et al, 1984;Archer et al, 2015), or plasma wave spectra (upper hybrid resonance) (Mosier et al, 1973;Moldwin et al, 2002;Thomas et al, 2021). These indirect techniques are complementary to each other.…”

Statistical Study on Small‐Scale (≤1,000 km) Density Irregularities in the Inner Magnetosphere

“…Therefore, this new method can exclude one-sided density structures, such as steep plasmapauses or plume boundaries. Additionally, our method performed well in excluding the medium-scaled azimuthal structures that are frequently observed near apogees (Thomas et al, 2021).…”

“…In situ statistical studies of density, irregularities include observations from geosynchronous satellites (Moldwin et al., 1995), CRESS spacecraft (LeDocq et al., 1994), Cluster missions (Darrouzet et al., 2004), and Van Allen probes (Thomas et al., 2021). These studies focused on small‐scale to medium‐scale density irregularities with cross‐field scales ranging from several hundred to several thousand kilometers.…”

“…These studies focused on small‐scale to medium‐scale density irregularities with cross‐field scales ranging from several hundred to several thousand kilometers. They found that larger density fluctuations are associated with stronger geomagnetic activity (Moldwin et al., 1995; Thomas et al., 2021) and with larger solar wind speeds during southward magnetic fields (Thomas et al., 2021). The largest fluctuation amplitudes are at L = 3–6 (LeDocq et al., 1994).…”

“…Therefore, we limited our statistics to small-scale density irregularities of which cross-field sizes are within the order of 1,000 km, and filtered out the medium-scale to large-scale structures such as plumes. In Thomas et al (2021)'s study, density perturbations over different scales were mixed. The scales of our targeted density irregularities correspond to typical observed ducts of whistler-mode waves with equatorial cross-field sizes of several tens to several hundreds kilometers (Angerami, 1970;Koons, 1989;Lester & Smith, 1980;Liu et al, 2021;Scarf & Chappell, 1973;Sonwalkar et al, 1994;Streltsov & Bengtson, 2020), and therefore, our results can provide insights into the ducted propagation of whistler-mode waves in the inner magnetosphere.…”