Defining the Middle Corona

Abstract

The middle corona, the region roughly spanning heliocentric distances from 1.5 to 6 solar radii, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. The solar wind, eruptions, and flows pass through the region, and they are shaped by it. Importantly, the region also modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, the middle corona is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the region has been poorly studied by both major solar remote-sensing and in-situ missions and instruments, extending back to the Solar and Heliospheric Observatory (SOHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions that it covers, and the underlying physics believed to shape the region. This article aims to define the middle corona, its physical characteristics, and give an overview of the processes that occur there.

Keywords: Corona.

Figure 1

A SWAP and LASCO composite image highlighting the middle corona, and the physical transitions that extend through the region. The image also highlights the observational gap between EUV observations of the inner corona and visible-light observations of the outer corona, currently experienced from the Earth perspective (e.g. Byrne et al., 2014). The image is annotated to highlight key heights, coronal characteristics, and physical transitions.

Figure 2

(a) Summary of past, present, planned, and proposed middle-corona observatories. The type of observation is indicated in parentheses, with key to symbolic abbreviations in upper right of the figure. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray). (b) Continuation of middle-corona observatories. The type of observation is indicated in parentheses, with key to symbolic abbreviations in upper right of the figure. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray). (c) Continuation of middle-corona observatories. Past and present instrumentation in order of first light. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray).

Figure 2

(a) Summary of past, present, planned, and proposed middle-corona observatories. The type of observation is indicated in parentheses, with key to symbolic abbreviations in upper right of the figure. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray). (b) Continuation of middle-corona observatories. The type of observation is indicated in parentheses, with key to symbolic abbreviations in upper right of the figure. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray). (c) Continuation of middle-corona observatories. Past and present instrumentation in order of first light. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray).

Figure 2

(a) Summary of past, present, planned, and proposed middle-corona observatories. The type of observation is indicated in parentheses, with key to symbolic abbreviations in upper right of the figure. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray). (b) Continuation of middle-corona observatories. The type of observation is indicated in parentheses, with key to symbolic abbreviations in upper right of the figure. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray). (c) Continuation of middle-corona observatories. Past and present instrumentation in order of first light. Color corresponds to the wavelength regime of the observation, X-ray (Gold), EUV/UV (Violet), Visible (Green), Infrared (Red), and Radio (Gray).

Figure 3

Different views of the middle corona, observed on 29 April 2021, in EUV from SUVI (top in 17.1 nm, and middle panel in 19.5 nm) and visible light from K-Cor (bottom left) and LASCO (bottom right; with SUVI superimposed). Images are in camera coordinates and not necessarily co-aligned, although solar North is generally upwards in each frame.

Figure 4

The proportion of the total emissivity contributed by resonant scattering as a function of height, for various fractions [$B$] of the model value of the solar-wind speed, as reported in Gilly and Cranmer (2020). $B$ is a scalar factor applied to the radial-wind-speed profile, with $B=0$ indicating no wind and $B=1$ indicating wind at nominal modeled values. Ion-line wavelengths are given in units of angstroms. (Figure 17 of Gilly and Cranmer, , used with permission).

Figure 5

A prominence eruption observed through the 30.4 nm passband of the EUI Full Sun Imager on 15 February 2022, when the Solar Orbiter spacecraft was located at 0.73 AU from the Sun, at 22:00 UT (top left), 22:04 UT (top right), 22:10 UT (middle left), 22:14 UT (middle right), 22:20 UT (bottom left), and 22:24 UT (bottom right). The observations have been processed using the radial-filtering technique described by Seaton et al. (2023) to enhance the off-limb signal, allowing the eruption to be tracked out to 5 ${\mathrm{R}}_{\odot }$. See Mierla et al. (2022) for further details about this event.

Figure 6

An example of how large FOV images can be processed to reveal structures extending into the middle corona. The three images show the same SWAP (17.4 nm) observation from 10 November 2014, processed nominally (top left), using a stacking technique (top right, see West et al. (2022) for further details), and using the MGN technique (bottom: Morgan and Druckmüller, 2014).

Figure 7

Intensity images of H i Ly$\alpha$ and O vi (103.2 nm) reconstructed from the sets of UVCS synoptic images 1 June 1996 through 3 June. Note the different morphologies above the west limb. Note that the units in the figure are given in angstroms.

Figure 8

Characteristic radio frequencies in the solar atmosphere. The middle corona includes a critical region where the transition of radio-emission mechanisms occurs. The dark-pink box marks the nominal range of the middle corona (≈ 1.5 – 6 ${\mathrm{R}}_{\odot }$) and the light-pink box marks an extended range taking into account the highly structured and dynamic nature of the corona. The corresponding frequencies that are relevant to radio observations of the middle-corona range from < 10 MHz to ≈ 1 GHz. (Adapted from Figure 4.1 in Gary and Hurford (2004) with permission.)

Figure 9

Overview of radio phenomena in the middle corona (A) Type-II burst with a well-defined split-band feature into an upper- and lower-frequency branch (UFB and LFB, respectively), which, if interpreted as plasma radiation from the shock upstream and downstream, can be used to estimate the shock-compression ratio and Mach number (Figure 2 in Mahrous et al. (2018), used with permission; see also Zimovets et al., 2012). (B) Type-III burst contours overlaid on an Solar Dynamics Observatory (SDO: Pesnell, Thompson, and Chamberlin, 2012)/Atmospheric Imaging Assembly (AIA: Lemen et al., 2012) 30.4 nm, image. Tracking the radio burst over several frequencies illustrates an evolution from a single source in the inner corona to two separate sources split between two separate flux tubes in the middle corona (Figure 14 in McCauley et al., , used with permission). (C) Type-IV burst associated with a radio CME resulting from trapped non-thermal electrons emitting gyrosynchrotron radiation, which can be used to determine the CME’s magnetic-field strength (Figure 2 in Carley et al., , used with permission). (D) The scintillation index (representing the magnitude of the intensity fluctuations) as a function of heliocentric distance; intensity scintillation provides information on the plasma density and solar-wind speed (Figure 3 in Imamura et al., , used with permission). (E) Frequency fluctuations provide information on plasma-density fluctuations and solar-wind speed. Upper panel shows raw frequency data dominated by a Doppler shift and the bottom panel shows the frequency fluctuations with the Doppler shift removed (Figure 2 in Wexler et al., , used with permission). (F) Faraday rotation provides information on the plasma density and magnetic-field component along the line of sight. Differences between measurements along two closely spaced lines of sight (provided here by a background radio galaxy) can be used to probe coronal electric currents (Figure 5 in Kooi et al., , used with permission).

Figure 10

Radial evolution for selected C, O, Fe ions within simulated coronal-hole wind, equatorial streamer-belt solar wind, adapted from Landi et al. (2012) and a CME adapted from Rivera et al. (2019). The horizontal dashed lines represent ions reaching 10% of their freeze-in value.

Figure 11

Ratio of modeled ion-line properties as a function of height, with and without including a modeled continuum in the resonantly scattered light. Panel a indicates excess line-width caused by including the continuum, while Panel b shows excess intensity. The shaded orange area indicates the middle-coronal region. Ion-line wavelengths are given in units of angstroms. Adapted from Figure 14 of Gilly and Cranmer (2020), and used with permission.

Figure 12

Two examples of forward modeling from the PSI eclipse predictions. Top panels: a merged image comparing the polarized brightness prediction for the 21 August 2017 total solar eclipse (a) to a processed eclipse photo (b, images adapted from Mikić et al., , and used with permission). Panel c shows radially filtered, sharpened radiances for the photoexcited Fe xi 789.2 nm emission line for the 14 December 2021 eclipse prediction (predsci.com/eclipse2021, see Boe et al., , for details on the method).