Source region identification and geophysical effects of stealth coronal mass ejections

Abstract

We use the example of two Stealth-type coronal mass ejections (stealth CMEs) registered by LASCO C2 on the quiescent Sun on 16 June 2010 and in the active region NOAA 11520 7 July 2012 to demonstrate that these mass ejections emerged in a lower-coronal area where an intense short-term radiation burst was registered in several extreme ultraviolet channels. It is established for the first time that the stealth CME emergence is accompanied by an eruption of small-scale loop-like structures, and the frontal structure (FS) formation is detected. The time dependences of the velocity of these FSs are discussed. The conclusion is made that having reached Earth orbit and impacting on the Earth magnetosphere, the stealth CMEs can lead to magnetic substorm generation.

Introduction

Most coronal mass ejections (CMEs) registered in a coronagraph field of view are related to various manifestations of solar activity in the lower corona (low coronal signatures — LCSs): flares, filament eruptions, EUV waves, dimming etc. Meanwhile, there are CMEs observed by coronapraphs that are not related to LCSs. It has been assumed for a long time that such CMEs emerge on the side of the Sun unseen by the observer. However, Robbrecht et al. (2009) demonstrated that CMEs unaccompanied by LCSs can emerge on the visible surface of the Sun. Later, LCSs-free CMEs began to be called ‘coronal mass ejections of the stealth type’ (stealth CMEs). Ma et al. (2010) compared the properties of CMEs with and without LCSs.

Howard and Harrison (2013) suggested that the absence of LCSs accompanying the emergence of CMEs could be related to low temporal and spatial resolution, as well as to low sensitivity of telescope sensors registering the solar radiation. This means that stealth CME formation is most likely to be accompanied by various activities in the lower corona (flares, filament eruptions etc.), but these activities cannot be registered by spacecraft onboard devices.

Today, most CMEs are believed to be triggered by filament eruption (over 80% of such eruptions result in CMEs) (Schmieder et al., 2013). As was noted above, however, no stealth CMEs are preceded by filament eruptions. Based on this, it can be suggested that the stealth CME generation mechanism differs from the generation mechanisms of most ‘ordinary CMEs’, but any details of this mechanism remain unknown. D’Huys et al. (2014) studied 40 events of stealth CMEs observed at 2012. Note that it has been the largest sampling of stealth CMEs examined to date. Despite the fact that the term ‘stealth CME’ came into being before (D’Huys et al., 2014), it was the authors of that paper who proposed to treat ‘stealth CMEs’ as coronal mass ejections emerging on the visible side of the Sun that are not accompanied by LCSs.

D’Huys et al. (2014) compared properties of the CMEs with LCSs and CMEs without LCSs. It was shown that an average speed of the listed stealth CMEs is equal about $300\text{ km/s}$. It is more less than an average linear speed of CMEs with LCSs of $435\text{ km/s}$. The angular widths of stealth CMEs turned out narrower ($26°$) than the angular widths of CMEs with LCSs ($\sim 40°$). And, finally, though the position angle (PA) of stealth CMEs can take any values within the $[0°-360°]$ range, but it is far more likely that $PA$ of many stealth CMEs is next to 0° (or 360° which is the same). One of the most difficult problems in the study of stealth CMEs is to prove that the observed CME appeared on the side of the Sun visible to the observer. D’Huys et al. (2014) proposed a method for determining the side of the Sun on which a CME was formed (the front or backside of the Sun relative to the observer), using principal angle of CME observed simultaneously in LASCO coronagraphs FOV and STEREO (A or B COR2 coronagraphs).

Alzate and Morgan (2017) used improved techniques for processing solar images in different spectral channels of the AIA/SDO telescopes and discovered that the formation of each stealth CME in D’Huys et al. (2014) was accompanied by certain small-scale manifestations of solar activity. They concluded that all 40 stealth CMEs from that list were associated with some form of LCSs (jets, or small filament eruptions, et al.). Alzate and Morgan (2017) concluded that including the mass ejections listed in D’Huys et al. (2014) into the stealth CME group was caused by limitations of space–time resolution of the telescope and preprocessing data methods. Efforts to register solar flares during the formation of mass ejections in D’Huys et al. (2014), in the integral flux of solar radiation, have however been unsuccessful. O’Kane et al. (2019) presented a study of two stealth CMEs analyzed using advanced image processing techniques that reveal their faint signatures in observations from the extreme-ultraviolet (EUV) imagers on board the SOHO, SDO and STEREO spacecrafts. Nor have Alzate and Morgan (2017) been able to register frontal structure (FS) formation for any stealth CME. It is necessary to recall that the CME FS is still the main indicator of initiation for any coronal mass ejection. While various other manifestations of LCSs should only be regarded as complementary indicators possibly observable during CME formation. It is difficult to identify any manifestations of solar activity in the lower solar corona, let alone small-scale ones involving CME formation, without detecting the CME FS. That is the reason why the issue of where the sources of many stealth CMEs are located in the Sun remains open, and developing new approaches to identifying the place and time of origin for stealth CMEs continues to remain a burning problem.

We surmised that the emergence of CMEs classified as belonging to the ‘stealth’ type must always be accompanied by some manifestations of solar activity, not only at the moment they are formed, but also after the fully-formed mass ejection begins to move. This can be explained by the fact that the coronal matter and magnetic field at the origin of any CME are perturbed. Registering the consequences of such perturbations can serve as the key to identifying the source of the stealth CMEs. The only difference that can be expected from stealth CMEs is the detection of some specific manifestations of LCSs, including small-scale LCSs, that are not observed for most ‘ordinary’ CMEs. Thus, even visual analysis of formation processes for many CMEs related to LCSs, based on ultraviolet (UV) solar images provides grounds to suggest that their emergence can be accompanied by various short-term small-scale activity. For example, in one of the first studies (Gallagher et al., 2003) authors shown that formation of a CME as large-scale structure was preceded by a short-term EUV brightening in several small-scale magnetic structures of the lower solar corona. We presumed that such a short-term small-scale solar activity can also accompany the emergence of stealth CMEs. On the other hand, the absence of powerful manifestations of solar activity (flares, filament eruptions etc.) in the lower corona rules out light-striking in various wavelength ranges, in the region where such mass ejections are formed. This means that we can examine the processes in this region more reliably. That is why we cannot exclude the possibility that studying just such events as stealth CMEs could allow us to better understand the formation mechanisms of ‘ordinary’ CMEs with LCSs.

Usually formation of CMEs with LCSs is also followed by reorganization of magnetic field configuration in the solar photosphere. Such magnetic field restructuring is often associated with emerging of a new magnetic flux (Feynman and Martin, 1995), active magnetic field canceling (Sterling et al., 2010) and others. It follows that formation and moving of stealth CMEs could lead to variations of photosphere magnetic field as well. As far as we know, to date this problem has not been discussed in literature and one of aims of our study is to fill this gaps.

Some researchers studied the formation stage of LCSs associated CMEs using data with high temporal and spatial resolution from SDO/AIA telescopes (e.g. Fainshtein and Egorov, 2015, Zagainova and Fainshtein, 2015, Grechnev et al., 2016). The authors detected FS of many CMEs in the lower solar corona and investigated FS kinematics in SDO FOV. But it is unknown whether it is possible to detect formation of a stealth CME FS. The second aim of this paper is to describe our approach to solving this problem.

It was established (Heber et al., 2015) when stealth CMEs approach the Earth they could cause Forbush decreases. Some researchers (e.g. Kilpua et al., 2014, Nitta and Mulligan, 2017) suppose that the geomagnetic field disturbances could be caused by a stealth CME effect on the Earth magnetosphere. He et al. (2018) showed that even a weak and low CME without LCSs detected on 8 October 2016 caused a relatively intense geomagnetic storm.

The aim of this paper is the presentation of a new approach to detect source location and initiation time of a stealth CME on the solar disk and case study of the stealth CMEs observed in LASCO C2 FOV on 2010 June 16 at 14:54:05 UT and 2012 Jule 7 at 18:12:05 UT. The selection of these two events for investigation is due to the following circumstances. First, our analysis showed that the sources of the above CMEs are far enough removed from the limb to allow a detailed analysis of the processes in the lower corona and the photospheric magnetic field accompanying the emergence of these coronal mass ejections. Moreover, the first of the CMEs we examine emerged on the disk of the quiescent Sun, i.e. during a period when sunspots were absent (even though spotfree active regions were observed on the Sun), the second stealth CME emerged during a period of strong solar activity in the active region with spots. Our approach to detect source location and initiation time of a stealth CME on the solar disk is based on detecting the formation of the FS of a stealth CME and delineating short-term radiation bursts in the EUV range from small-scale sources as well as the eruptions of small-scale magnetic flux-ropes observed in this range, at the FS origin. We regard this activity as a characteristic LCS accompanying the emergence of stealth CMEs. Note that we have been the first to discover the FS formation of stealth CMEs. The response of the magnetosphere and the magnetic field of the Earth to these stealth CME are discussed as well. The properties of these two stealth CMEs were partially discussed in our papers (Zagainova and Fainshtein, 2019, Zagainova et al., 2019).