Statistical analysis of the mid-latitude ionospheric trough

Introduction

  • The mid-latitude ionospheric trough is a region of depleted plasma densities in the F-region observed persistently at the sub-auroral latitudes.
  • The  is completely understood.
  • Two important factors in the region are expected to influence formation of trough :
    • ​Stagnation of flows (in the region between inner and outer magnetosphere).
    • Strong electric fields (SAPS ?).
  • The goal of this study is to develop a statistical characterization of the trough and its relation with the mid-latitude trough.

Automatic detection of the mid-latitude trough

Identifying the mid-latitude trough

  • Figure presents median filtered (Evan's method) GPS TEC data (Apr 9, 2011 at 0900 UT) [Clausen et al., 2011].

  • The boundaries of the trough and min trough location are automatically identified using the method described before.

  • We automatically detected troughs during SAPS periods between 2011 and 2014. The trough detection technique worked well for quite a few examples and didn't for some.

Trough occurrence statistics during SAPS periods.

  • Figure, presents normalized data counts at different MLTs and Dst-bins, where the trough was identified.

  • For example, trough was successfully identified the most number of times (100%) near -1 (23) MLT when Dst is between -10 and 10 nT.

  • The peak trough location moves duskwards with increasing geomagnetic activity.

  • The pattern is similar to the one observed with SAPS location.

Mid-latitude Trough grouped by Dst

  • Mean location and TEC values of trough boundaries and minimum location grouped by Dst.

  • Trough location moves equatorward and duskwards with increasing geomagnetic activity.

Variations in TEC values with MLT

  • Mean TEC values at trough minimum vs MLT at different Dst levels.

  • No pattern in particular, TEC values are higher near dusk (day-night terminator ?).

  • Mean TEC values are between 4 and 5 for all Dst bins near mid-night and some what higher near dusk.

Trough location with respect to SAPS

  • Figure presents SAPS prob of occurrence plots ( Kunduri et al [2017] ) overlaid along with trough edges and minimum location.

  • Although takes from different datasets, SAPS and the mid-latitude trough show a great degree of alignment.

  • Also, interestingly for bins where Dst > -25 nT, SAPS is located between minimum and poleward edge. For higher disturbance levels SAPS is located between both the edges.

Equatorward trough depth with Dst-index

  • Figure presents mean difference between  TEC at equatorward edge of trough and trough min.

  • Generally, the trough is deeper towards dusk and at disturbed geomagnetic activity levels.

  • A deeper trough is associated with stronger SAPS electric fields (SAPS velocities are stronger towards dusk and at more negative Dst levels) ?

Poleward trough depth with Dst-index

  • Difference between TEC at poleward edge of trough and trough min.

  • The trough is not as deep as the one observed on the equatorward edge. Missing data at higher latitudes in the candian sector?

  • Contrary to equatorward edge, appears to be slightly deeper moving away from dusk.

Trough Occurrence Probability model

  • Actual trough observations (blue circles) and Gaussian fitted curve (red line).
  • trough_prob = 0.98*exp( -(normMLT+0.38-0.03*dst)^2/(-6.52-0.011*dst)^2  )

Trough location model

  • Actual trough location observations (blue) and fitted curves (red).
  • The MLATs were fitted as exponential function of MLT.

Conclusions and future work

  • Statistically demonstrated trough is collocated with SAPS in general
  • Deeper troughs are associated with higher SAPS fields - confirming the feedback mechanism?
  • Developed a SAPS time trough location model.
  • Future work :
    • Discuss the trough detection method with others?
    • Similar analysis for non-SAPS periods.
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