Ionospheric storms at geophysically-equivalent sites – Part 2: Local time storm patterns for sub-auroral ionospheres

M Mendillo,C Narvaez

doi:10.5194/angeo-28-1449-2010

Abstract

Abstract. The response of the mid-latitude ionosphere to geomagnetic storms depends upon several pre-storm conditions, the dominant ones being season and local time of the storm commencement (SC). The difference between a site's geographic and geomagnetic latitudes is also of major importance since it governs the blend of processes linked to solar production and magnetospheric input, respectively. Case studies of specific storms using ionospheric data from both hemispheres are inherently dominated by seasonal effects and the various local times versus longitude of the SCs. To explore inter-hemispheric consistency of ionospheric storms, we identify "geophysically-equivalent-sites" as locations where the geographic and geomagnetic latitudes have the same relationship to each other in both hemispheres. At the longitudes of the dipole tilt, the differences between geographic and geomagnetic latitudes are at their extremes, and thus these are optimal locations to see if pre-conditioning and/or storm-time input are the same or differ between the hemispheres. In this study, we use ionosonde values of the F2-layer maximum electron density (NmF2) to study geophysical equivalency at Wallops Island (VA) and Hobart (Tasmania), using statistical summaries of 206 events during solar cycle #20. We form average patterns of ΔNmF2 (%) versus local time over 7-day storm periods that are constructed in ways that enhance the portrayal of the average characteristic features of the positive and negative phases of ionospheric storms. The results show a consistency between four local time characteristic patterns of storm-induced perturbations, and thus for the average magnitudes and time scales of the processes that cause them in each hemisphere. Subtle differences linked to small departures from pure geophysical equivalency point to a possible presence of hemispheric asymmetries governed by the non-mirror-image of geomagnetic morphology in each hemisphere.

Highlights

Perturbations to the Earth’s ionosphere arise from many different sources
In Paper-1, we explored these effects via subsets of storms sorted by season, the local time of the storm commencement (SC), the phase of the solar cycle, and the severity of the storm
To examine possible influences of geomagnetic latitude, we looked at the pattern of ionospheric storm effects that occur during years of declining solar cycle conditions from the station at Christchurch, New Zealand (43.6◦ S, 172.8◦ E)

Summary

Introduction

Perturbations to the Earth’s ionosphere arise from many different sources. The most dramatic and well documented effects are those that occur during periods of global geomagnetic storms. The resultant “ionospheric storms” have been studied using several types of observations of the F2layer: (a) ionosonde values of the maximum electron density (NmF2), (b) satellite radio beacon measures of total electron content (TEC), (c) incoherent scatter radar (ISR) measures of electron and ion densities, temperatures and plasma dynamics, and (d) satellite in situ measures of ionosphere/thermosphere parameters along orbital tracks. Comprehensive summaries of storm effects in NmF2 and in TEC have been given by Prolss (1995) and Mendillo (2006), respectively, for ISR results by Buonsanto (1999), and in satellite data by Prolss (1974). The basic morphologies of ionospheric storms within different latitude zones, and the processes that drive them, have been identified and in many cases modeled successfully. This brief summary refers to the study of storm effects in the Northern Hemisphere

Objectives

Results

Discussion

Conclusion