The Dipolar Solar Minimum Corona

Daniele Telloni

doi:10.3390/universe7120507

Abstract

The large-scale configuration of the UV solar corona at the minimum activity between solar cycles 22 and 23 is explored in this paper. Exploiting a large sample of spectroscopic observations acquired by the Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory in the two-year period of 1996–1997, this work provides the first-ever monochromatic O vi 1032 Å image of the extended corona, and the first-ever two-dimensional maps of the kinetic temperature of oxygen ions and the O vi1037/1032 Å doublet intensity ratio (a proxy for the outflow velocity of the oxygen component of the solar wind), statistically representative of solar minimum conditions. A clear dipolar magnetic structure, both equator- and axis-symmetric, is distinctly shown to shape the solar minimum corona, both in UV emission and in temperature and expansion rate. This statistical approach allows for robust establishment of the key role played by the magnetic field divergence in modulating the speed and temperature of the coronal flows, and identification of the coronal sources of the fast and slow solar wind.

Highlights

The solar corona is the outermost layer of the Sun’s atmosphere
Looking at the results reported in the figures of the previous section as a whole, the first observational evidence that clearly emerges is the strikingly prominent dipolar structure of the solar minimum corona, characterized by large polar coronal holes and an equatorial streamer belt
This paper provides the first 2D maps of the UV solar corona that are statistically significant and representative of the solar minimum of solar cycle (SC) 22/23

Summary

Introduction

The solar corona is the outermost layer of the Sun’s atmosphere It is the region where the plasma is heated to such high temperatures that it is much hotter than the underlying solar surface in such a seemingly paradoxical fashion [1]. It is where the solar wind, a continuous stream of charged particles, originates and is accelerated to supersonic speeds, expanding into the heliosphere, the Sun’s region of influence that permeates the whole solar system and a bit beyond, interacting with the planets’ magnetospheres [2]. From the launch of the Solar and Heliospheric Observatory (SOHO, [13]) on 2 December 1995, the solar corona has been being monitored daily thanks to the Large Angle Spectrometric COronagraph (LASCO, [14]) instrument suite (which comprises three visible-light coronagraphs) and to the Ultraviolet Coronagraph Spectrometer (UVCS, [15]), capable of measuring the speed of the outflowing coronal plasma in its early phase of propagation towards the heliosphere

Objectives

Discussion

Conclusion