Ionospheric Space Weather

Longitude Dependence and Lower Atmosphere Forcing

Timothy Fuller-Rowell, Endawoke Yizengaw, Patricia H. Doherty, Sunanda Basu, Timothy Fuller-Rowell, Endawoke Yizengaw, Patricia H. Doherty, Sunanda Basu

• Publisher: American Geophysical Union
• Year: 2016
• Print ISBN: 9781118929209
• eBook ISBN: 9781118929230
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geophysics

This monograph is the outcome of an American Geophysical Union Chapman Conference on longitude and hemispheric dependence of ionospheric space weather, including the impact of waves propagating from the lower atmosphere. The Chapman Conference was held in Africa as a means of focusing attention on an extensive geographic region where observations are critically needed to address some of the fundamental questions of the physical processes driving the ionosphere locally and globally. The compilation of papers from the conference describes the physics of this system and the mechanisms that control ionospheric space weather in a combination of tutorial-like and focused articles that will be of value to the upper atmosphere scientific community in general and to ongoing global magnetosphere-ionosphere-thermosphere (MIT) modeling efforts in particular. A number of articles from each science theme describe details of the physics behind each phenomenon that help to solve the complexity of the MIT system. Because this volume is an outcome of the research presented at this first space science Chapman Conference held in Africa, it has further provided an opportunity for African scientists to communicate their research results with the international community. In addition, the meeting and this conference volume will greatly enhance the space science education and research interest in the African continent and around the world.

Ionospheric Space Weather includes articles from six science themes that were discussed at the Chapman Conference in 2012. These include:

• Hemispherical dependence of magnetospheric energy injection and the thermosphere-ionosphere response
• Longitude and hemispheric dependence of storm-enhanced densities (SED)
• Response of the thermosphere and ionosphere to variability in solar radiation
• Longitude spatial structure in total electron content and electrodynamics
• Temporal response to lower-atmosphere disturbances
• Ionospheric irregularities and scintillation

Ionospheric Space Weather: Longitude Dependence and Lower Atmosphere Forcing will be useful to both active researchers and advanced graduate students in the field of physics, geophysics, and engineering, especially those who are keen to acquire a global understanding of ionospheric phenomena, including observational information from all longitude sectors across the globe.

Part I
Hemispherical Dependence of Magnetospheric Energy Injection and the Thermosphere‐Ionosphere Response

1
Interhemispheric Asymmetries in Magnetospheric Energy Input

Eftyhia Zesta,¹ Athanasios Boudouridis,² James M. Weygand,³ Endawoke Yizengaw,⁴ Mark B. Moldwin,⁵ and Peter Chi⁶

¹Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

²Center for Space Plasma Physics, Space Science Institute, Boulder, Colorado, USA

³Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California, USA

⁴Institute for Scientific Research, Boston College, Chestnut Hill, Massachusetts, USA

⁵Atmospheric, Oceanic, and Space Science (AOSS), University of Michigan, Ann Arbor, Michigan, USA

⁶Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, California, USA

ABSTRACT

Energy transfer from the solar wind to the magnetosphere‐ionosphere‐thermosphere system occurs via multiple routes with coupling efficiency depending on the Interplanetary Magnetic Field (IMF), solar wind, and the magnetosphere prior state. The energy is not always released in the two hemispheres symmetrically. Ultra low frequency (ULF) waves are the natural perturbations of the magnetosphere and the plasma in it, thus constituting an excellent diagnostic of how energy is transported throughout this complex system. We explore the question of how energy is deposited asymmetrically in the two hemispheres by studying (1) asymmetries of auroral currents and (2) asymmetries in ULF wave power at magnetically conjugate locations. We also construct a Southern Hemisphere auroral electrojet (AE) index and compare it with the standard AE index. We find that while in general the north and south electrojet indices correlate well, significant asymmetries occur frequently, primarily in the local midnight region. We also find that at low latitudes and midlatitudes the north‐to‐south wave‐power ratio exhibits clear annual variation with a systematic offset: the Northern Hemisphere always has stronger power than the Southern Hemisphere. This systematic asymmetry is also seen in the ionospheric total electron content (TEC), implying a close link.

Key Points:

Interhemispheric asymmetries in ULF wave power and total electron content
A southern auroral electrojet index and comparison with the standard AE index
Interhemispheric asymmetries between northern and southern auroral electrojet indices

Key Terms: equatorial ionosphere, equatorial electrojet (EEJ), ground‐induced currents (GIC)

1.1. INTRODUCTION

It is generally assumed that most of the dynamic geospace phenomena, like magnetic storms and substorms, develop in unison in both Northern and Southern Hemispheres, typically starting in the polar regions. High‐latitude geomagnetic field lines carry a load of field‐aligned currents (FACs) and electromagnetic waves directly from the magnetopause, where the heavy coupling from the solar wind to the magnetosphere occurs, down to the ionosphere and thermosphere, depositing energy in the form of Poynting flux that heats both the ionosphere and neutral atmosphere. A part of the solar‐wind energy gets processed in the magnetotail first, and is ultimately deposited in the ionosphere via both currents and electromagnetic waves, but also particle precipitation that can form bright auroras. Another part of the solar‐wind energy is stored in the inner magnetosphere and couples to the midlatitude and low‐latitude ionosphere through electric fields, waves, and particle precipitation. During equinox, it is generally assumed that the load of currents and waves is approximately symmetric into the north and south polar ionospheres, but becomes quite asymmetric when either of the poles is tilted toward the Sun during the solstices [e.g., Wu et al., 1991]. At those times, the uneven solar EUV illumination becomes a controlling factor for the asymmetric ionospheric conductivity in the two polar regions, leading to large asymmetries in the electrodynamic coupling with the magnetosphere and the amount of heating that is transferred to the neutrals.

While seasonal effects are strong drivers of interhemispheric asymmetries, other factors, such as the dipole tilt with respect to the rotation axis, the Interplanetary Magnetic Field (IMF) orientation, local magnetic field structures, and even atmospheric dynamics, can and do play a significant role in the strong interhemispheric asymmetries that are observed at all latitudes. For example, Knipp et al. [2000] showed significant difference in the amount of energy input, both from Joule heating and precipitation, in the two hemispheres during an 11‐hr interval in May 1999. Knipp et al. argued that the large asymmetries were due to both the Northern Hemisphere sunward tilt and to the IMF orientation.

The tilt and offset of the dipolar part of the Earth’s magnetic field places the polar caps at different geographic latitudes resulting in lower geomagnetic latitudes seeing 24 hr darkness in the Southern Hemisphere in the Americas longitude sector than in the Northern Hemisphere during northern summer, further exacerbating conductivity and electrodynamic asymmetries. Cnossen and Richmond [2012] demonstrated via modeling that the tilt angle of the geomagnetic dipole is a strong controlling factor in the distribution of Joule heating in the high latitudes and in the neutral temperature and winds. Förster and Cnossen [2013] took this work further to demonstrate, again via modeling, the effect the nondipolar components of the Earth’s magnetic...

Table of contents

• Cover
• Title Page
• Table of Contents
• CONTRIBUTORS
• PREFACE
• Part I: Hemispherical Dependence of Magnetospheric Energy Injection and the Thermosphere‐Ionosphere Response
• Part II: Longitude Dependence of Storm‐Enhanced Densities (SEDs)
• Part III: Longitude Spatial Structure in Total Electron Content and Electrodynamics
• Part IV: Temporal Response to Lower Atmosphere Disturbances
• Part V: Response of the Thermosphere and Ionosphere to Variability in Solar Radiation
• Part VI: Ionospheric Irregularities and Scintillation
• INDEX
• End User License Agreement