A Statistical Study of Near‐Earth Magnetotail Evolution During Pseudosubstorms and Substorms With THEMIS Data

Abstract

Pseudosubstorms (pseudobreakups) and substorms are similar phenomena. In terms of auroral morphology, however, the former are not accompanied by poleward expansion, while the latter are. To understand what causes this difference, we studied temporal and spatial development of the near-Earth magnetotail at X = −7 to −11R E around pseudosubstorm and substorm onsets, based on superposed epoch analysis of Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. We find that the earthward flow begins to increase at X = −10 to −11R E just before onset and dipolarization for both pseudosubstorms and substorms, possibly due to near-Earth magnetic reconnection or the preceding relaxation of the thin current sheet in a tailward region, but the earthward flow is slower for pseudosubstorms than for substorms. Dipolarization, together with magnetic field fluctuation, is nearly the same at X = −8R E for both cases, but it is weaker at other distances for pseudosubstorms than for substorms. This result suggests that the current disruption related to dipolarization does not expand tailward and hence auroral poleward expansion does not occur for pseudosubstorms. Furthermore, the total pressure is larger at X = −8 to −11R E for several minutes before onset for substorms than for pseudosubstorms. The total pressure gradient increases more largely after onset for substorms than for pseudosubstorms. We suggest that these differences are important factors for determining whether ballooning instability causing current disruption grows in a wide area, that is, whether the initial action develops into a substorm or subsides as a pseudosubstorm.

Statistical Characterization of the Dynamic Near‐Earth Plasma Sheet Relative to Ultra‐Low Frequency (ULF) Wave Growth at Substorm Onset

“…A similar result was reported by Fukui et al. (2019) in their survey of the near‐Earth magnetotail during the growth phase of substorms. Fukui et al.…”

“…We find that the magnitude of the flow is dependent upon location within the magnetotail, with the peak flows being up to 100% faster in the mid-tail (−10 ≤ X GSM ≤ −12.5 R E ) than in the near-tail (−7.5 ≤ X GSM ≤ −10 R E ). A similar result was reported by Fukui et al (2019) in their survey of the near-Earth magnetotail during the growth phase of substorms. Fukui et al (2019) attributed these flows and observations to either near-Earth magnetic reconnection (e.g., Angelopoulos et al, 2008), or the relaxation of the current sheet toward the Earth around substorm onset (e.g., Machida et al, 2009Machida et al, , 2014.…”

“…The strong earthward flows (in 𝐴𝐴 𝐴𝐴 GSM ⟂,𝑋𝑋 ) observed appear to subside in an oscillatory manner, with periods of the order of ∼5 min. Similar observations were also made by Fukui et al (2019), and could represent the bouncing of the earthward flows (e.g., Ohtani et al, 2009;Panov et al, 2010). While the mid-tail observations (−10 ≤ X GSM ≤ −12.5 R E ) showed stronger flows, we also see that these subside more slowly and show greater variability.…”

“…Fukui et al. (2019) attributed these flows and observations to either near‐Earth magnetic reconnection (e.g., Angelopoulos et al., 2008), or the relaxation of the current sheet toward the Earth around substorm onset (e.g., Machida et al., 2009, 2014). Assuming an Earthward velocity of 80 kms −1 , any flow resulting from reconnection at ∼20 R E would require 13–14 min to travel 10 R E to the location of the spacecraft.…”

A Case Study of Near‐Earth Magnetotail Conditions at Substorm and Pseudosubstorm Onsets

“…In particular, we examined in detail the near-Earth magnetotail at X ∼ −10 R E just before and after onset, including the pressure gradient projection along two closely located spacecraft. Besides this case study, we performed a statistical study and obtained similar results (Fukui et al, 2017), but we would like to report the details in a separate paper.…”