High-intensity, long-duration, continuous AE activity events associated with Alfvénic fluctuations in 2003

Alan Prestes,Arian Ojeda-González,Virginia Klausner

doi:10.5194/angeo-35-1231-2017

Abstract

Abstract. The interaction between a fast-speed and a low-speed stream causes large-amplitude Alfvénic fluctuations; consequently, the intermittency and the brief intervals of southward magnetic field associated with Alfvén waves may cause high levels of AE activity, the so-called high-intensity, long-duration, continuous AE activity (HILDCAA). In this article, the 4 h windowed Pearson cross-correlation (4WPCC) between the solar wind velocity and the interplanetary magnetic field (IMF) components is performed in order to confirm that the less strict HILDCAA (HILDCAAs*) events include a larger number of Alfvén waves than the HILDCAA events, once HILDCAAs disregard part of the phenomenon. Actually, a HILDCAA event is entirely contained within a HILDCAA* event. However, the opposite is not necessarily true. This article provides a new insight, since the increase of Alfvén waves results in an increase of auroral electrojet activity; consequently, it can cause HILDCAAs* events. Another important aspect of this article is that the superposed epoch analysis (SEA) results reaffirm that the HILDCAAs* are associated with high-speed solar streams (HSSs), and also the HILDCAAs* present the same physical characteristics of the traditional HILDCAA events.

Highlights

During the declining phase of solar cycles, the dominant solar phenomena affecting the geomagnetic activity are coronal holes (Krieger et al, 1973; Hapgood, 1993; Webb, 1995; Tsurutani et al, 2006), which are characterized by exceptionally low densities and unipolar photospheric magnetic fields with “open” magnetic field topologies
The declining phase of the cycle 23, especially the year of 2003, was exceptional in producing intense and continuous AE geomagnetic activity because there were more than two corotating streams present in each month as observed here
A great number of HILDCAA* events occurred in 2003, and this fact may be explained by solar energy transfer into the magnetosphere by the Alfvén waves

Summary

Introduction

During the declining phase of solar cycles, the dominant solar phenomena affecting the geomagnetic activity are coronal holes (Krieger et al, 1973; Hapgood, 1993; Webb, 1995; Tsurutani et al, 2006), which are characterized by exceptionally low densities and unipolar photospheric magnetic fields with “open” magnetic field topologies. Prestes et al (2017) included a small modification in the following criterion: “the AE values should never drop below 200 nT for more than 2 h at a time”, by changing 2 to 4 h at a time, in order to increase the number of HILDCAA events available for study. They defined these slightly modified events as HILDCAAs* (see Prestes et al, 2017, for more details). We expect that these correlations will be higher during HILDCAA and HILDCAA* events, since Alfvén waves usually induce continuous auroral zone activity

Datasets and methodology

Results and discussion

Conclusions