Multi-wavelength Observations of Two Explosive Events and Their Effects on the Solar Atmosphere

Abstract

We investigated two flares in the solar atmosphere that occurred on June 3, 2012 and July 6, 2012 and caused propagation of Moreton and EIT waves. In the June 3 event, we noticed a filament winking which presumably was caused by the wave propagation from the flare. An interesting feature of this event is that there was a reflection of this wave by a coronal hole located alongside the wave propagation, but not all of this wave was transmitted by the coronal hole. Using the running difference method, we calculated the speed of Moreton and EIT waves and we found values of 926 km/s before the reflection and 276 km/s after the reflection (Moreton wave) and 1,127 km/s before the reflection and 46 km/s after the reflection (EIT wave). In the July 6 event, this phenomenon was accompanied by type II and type III solar radio bursts, and we also performed a running difference analysis to find the speed of the Moreton wave, obtaining a value of 988 km/s. The speed derived from the analysis of the solar radio burst was 1,200 km/s, and we assume that this difference was caused by the different nature of the motions in these phenomena, where the solar radio burst was caused by the propagating particles, not waves.

1. INTRODUCTION

There are many events in the solar atmosphere that are considered as explosive events, and usually they have some impacts that can be detected on Earth (Daglis 2001; Bothmer & Daglis 2007). Although these phenomena have been studied extensively, there are still some unsolved problems. Flares, coronal mass ejections, and coronal holes are some of the phenomena that could exert considerable influence toward the Earth. Often, these eruptive processes trigger waves that propagate across the solar surface and can be observed in several wavelengths, mainly in Hα and extreme ultraviolet. The first observation of these waves was conducted in Hα wavelength by Moreton (1960). Subsequent observations revealed that such waves propagate with velocities of 500-1,500 km/s with angular extent of 90-270° (Warmuth et al. 2004; Balasubramaniam et al. 2010). Moreton waves usually occur after a large solar flare (Moreton 1960; Athay & Moreton 1961), where the flare explosion triggers a shock wave that propagates along the chromosphere.

Uchida (1968) modeled the Moreton wave mechanism. In his model, a solar flare initiates a coronal shock disturbance that propagates as a hydromagnetic fast-mode wave whose front has a circular intersection line with the upper chromosphere. The chromospheric (Moreton) wave is then treated as an acoustic wave due to the refraction of the coronal wave at this intersection. Since Moreton waves were observed on only one side of the flare, Uchida suggested that this asymmetry was due to a limited-directivity explosion of the flare. Nevertheless, there are some arguments that go against this because there were some flares that occur with no Moreton waves observed. Cliver et al. (1999) argued that the source of the Moreton waves is coronal mass ejections, not flares.

The advent of space-borne observations enabled solar physicists to look beyond the optical window to the ultra violet and extreme ultra violet, and to see that there are similar waves in the extreme ultra violet band. These waves are called EIT (extreme ultraviolet imaging telescope) waves because they were first observed using the EIT onboard the SOHO spacecraft in 195 nm wavelength (Zhukov et al. 2009). The speed and angular extent of the EIT waves are different from those of Moreton waves. EIT waves propagate with speed of about 170-350 km/s (Klassen et al. 2000). Some other researchers observed that the EIT propagates with speed in a range of 200-400 km/s with an average of 270 km/s (Wills-Davey & Attrill 2009). The angular extent of these waves is usually wider than that of the Moreton waves, being somewhat isotropical (Asai et al. 2012). EIT waves occur more frequently than Moreton waves, and accordingly have been more extensively studied. In addition, in contrast with Moreton waves, EIT waves can be observed through many channels using the atmospheric imaging assembly on the solar dynamic observatory (SDO/AIA, Lemen et al. 2012), whereas Moreton waves can be observed in the Hα channel only.

The observational differences between Moreton and EIT waves have sustained arguments about the nature of these waves. Although Moreton waves are observed in Hα, Balasubramaniam et al. (2007) argued that these waves propagate in the corona, not in the chromosphere. Furthermore, the large discrepancies between the angular extent of the Moreton waves with that of EIT waves indicates that these waves are very different, both in terms of the mechanisms and the nature of their propagation. At present, the nature of Moreton and EIT waves is still a source of heated debate. The main points of the debate are whether the EIT waves are:

In some cases, the Moreton and EIT waves interacted with coronal holes during their propagation. These interactions sometimes are used to prove the nature of these waves as a real wave. Olmedo et al. (2012) investigated a X2.2 flare and coronal mass ejection that occured on February 15, 2012. They found that the wave that passed through the coronal hole was accelerated and from this they inferred that this wave is a real wave that obeys the law of reflection and transmission through a medium. Similar proof was provided by Gopalswamy et al. (2009) when they analyzed an EUV wave that was related with a fast CME (speed ~950 km/s) and a long-duration flare (GOES X-ray class B9.5) that occured on May 19, 2007 from the active region NOAA 10956. This phenomenon occured near a coronal hole and they found that the resulting EUV wave from this flare interacted with this coronal hole.

The speed analysis of the EUV waves showed that there was a change of speed and reflection of this wave after encountering the coronal hole. They argued that the speed change and reflection of the wave support the notion that the EUV wave is a real wave, not a signature of a propagating perturbation related to magnetic field line opening in the wake of the associated CME (Veronig et al. 2006). They also found that the resulting wave from the flare interacted with a coronal hole was decelerated by the coronal hole and they also proved that the wave obeys the laws of reflection and transmission and hence can be considered a real wave.

This paper reports our investigation of two Moreton waves, where one is accompanied with an EIT wave. We consider these two waves in detail because they were very different from each other and seek to gain some insight into this elusive wave from this very marked difference. These two events occurred on June 3 and July 6, 2012, respectively, soon after the occurrence of M3.3 flare (June 3, 2012) and X1.1 flare (July 6, 2012) and coronal mass ejections. The phenomena that occurred on July 6 were accompanied by type II and III solar radio bursts. Preliminary analyses regarding these events have been reported in Admiranto & Priyatikanto (2015) and Admiranto et al. (2014) while the present paper presents a thorough analysis. Section 2 reviews the multi-wavelength data to be analyzed. Section 3 discusses the data analysis and the results of this analysis. Section 4 provides a discussion of the results, and a summary is given in Section 5.

2. DATA

In this study, we focus on two flaring events that occurred on June 3, 2012 and July 6, 2012. The first event occurred at NOAA 11496 (N16E38) and reached maximum X-ray flux of M3.3 at 17:55 UT. The second event occurred on July 6, 2012 and occurred in the active region NOAA 11515 (S17W41) with maximum X-ray flux of X1.1 at 23:01 UT.

To better understand the Moreton wave resulting from those events, we analyzed a number of Hα images from the global oscillation network group (GONG, Harvey et al. 1996) coordinated by the National Solar Observatory. The images were recorded with time resolution of about 20 seconds. In total, we use 36 images for the first event and 30 images for the second. We have used GONG data because the network includes many observatories to observe the chromosphere in high cadence so that the data completeness is more reliable. A couple of images are not of high quality and propagating wave features were not clearly detected. We chose only the data that can be used in our running difference analysis (will be described below).

EUV images from SDO/AIA (Lemen et al. 2012) were used to investigate the behavior of EIT waves that occur in the corona. We used images from the 304 nm channel because this channel gave the most pronounced features, and hence the running difference analysis was much easier. Additionally, to support our analysis, we also used coronal hole images obtained from the 171 nm channel.

3. ANALYSIS

3.1 Moreton Wave of June 3

From the Hα images, we obtained some bright features that usually occur accompanying a flare. After the flare started, some bright filaments occured near the flare location. We followed the evolution of the active region since the beginning of the flare. We noticed several interesting features in these image sequences during the progression toward its maximum (Fig. 1). From this data, we can clearly see the moving density enhancement which can be interpreted as a wave that moved in the chromospheric region. From the movie of the sequence of Hα images, we also noticed a winking filament which can be attributed to the motion of the wave emanating from the flare location.

Fig. 1.Sequence of Hα images of NOAA 11496 region. There is progression of brightness enhancements that occurred before the flare explosion, and one can also see the filament winking (arrow), which was presumably caused by a wave propagation.

We made a running difference analysis from the Hα and one can see the filament moving away from the center of the flare site. Looking at a sequence of pictures and making an animation of those pictures we can see that after the occurence of the bright features there was a propagating brightness enhancement that can be interpreted as a propagation of a wave. Fig. 1 shows the wave motion and we can see that this wave spanned angle of about 40°. From this motion, we also derived the speed of the wave propagation based on the position of the wave in time. The diagram below (Fig. 2) depicts the position of the wavefronts versus time, and from this diagram, we derived the speed of the Moreton wave as 988 ±70 km/s.

Fig. 2.Radial distance of the wavefronts versus time derived from the running differences of Hα and EIT images. One can see the abrupt change of speed in both the Moreton wave and EIT the wave. Filled and empty symbols represent the measured value from Hα and EUV observations, respectively. Triangles and circles represent the observed propagated and reflected waves.

3.2 EIT Wave of June 3

Observational data in EIT waves were obtained from SDO in several wavelengths, i.e. 94, 131, 193, 211, 304, and 335 Å, but more clear features can be observed in 304 Å. The running differences analyses was conducted for the acquired SDO images. This technique will enhance any subtle differences between two consecutive images, and thus any motion of material caused by the flare can be easily detected. We tried to obtain the running difference between two images in which the time distance between consecutive images is 5 min to gain insight into the dynamics of the filament, and we found the differences in images taken from 17:48 UT till 18:18 UT.

If we look closely at the results of this technique, which was applied to the active region images, some filament motion began at about 17:53 UT and this motion ended at about 17:58 UT. The beginning of the filament motion occured later than the beginning of the flare, and it ended just after the maximum time of the flare.

As seen in Fig. 3, the running differences technique employed for these images reveals some interesting features, there were some brightness changes in the images and presumably some reflection of wave caused by a coronal hole located at the right of the flare explosion. From the images shown in Figs. 3(c) and 3(d), it appears that the filament was moved upwards and hence this give the impression that the coronal hole reflected the incoming wave caused by the flare.

Fig. 3.Running differences for images taken between 17:48 UT and 18:18 UT with 5 min time differences. Note the upward motion of the filament (dashed arrow) and the propagated wave (solid arrow).

To obtain the effect of the flare explosion toward its surrounding, we tried to obtain a broader view of the Sun and made a running difference analysis of the images taken consecutively from 17:53 UT till 17:58 UT with a time step of 1 min, and the results are shown in Fig. 4. These images were adjusted in terms of contrast and brightness, and hence the subtler image differences can be more clearly distinguished.

Fig. 4.Running differences of two consecutive images. The contrast and brightness were adjusted between 17:53 UT-17:58 UT. The motion of the wave is observed in two directions.

From the images, one can see that there are two waves that originated from the flare explosion, and closer inspection reveals that these two waves were related to the coronal hole; this coronal hole reflected and tranmitted the incoming wave. This reflection and transmission of the wave was proven in speed analysis of the waves. As clearly seen in Fig. 2, the speed of the transmitted wave is much less than that of the reflected wave, and it appears that the energy of the transmitted wave was absorbed by the coronal hole.

An interesting feature of the wave observed in these wavelengths is the existence of a coronal hole by which the wave was reflected. The speed and direction of the EIT wave are influenced by this coronal hole, as shown in Fig. 5. It appears that the two movements, as observed in Fig. 4, were caused by the existence of this coronal hole, in which one part of the wave was reflected by the coronal hole and the other part was transmitted with reduced velocity.

Fig. 5.The movement direction of a wave reflected by the coronal hole near the flare location(Courtesy of www://helioviewer.org).

3.3 Moreton Wave of July 6

From Fig. 6 we conclude that the winking filament that can be observed in the June 3 phenomenon was not present in this event. On the other hand, we see a portion of wave motion caused by the flare explosion, which caused some brightness changes, as can be seen in the Fig. 6 sequence.

Fig. 6.Running difference of inverted Hα images. Note the arcs that delineate the wave motion in the chromosphere.

As done for the June 3 phenomenon, we try to analyze the speed of the wave motion caused by the flare. To this end, we cropped and inverted the images and performed a running average analysis to enhance the differences among the images so that the wave motion can be detected and computed more easily. Fig. 6 below shows the results of running difference analysis for inverted images in the region.

Using these images, we made a plot of location versus time to obtain the speed of wave propagation along the chromosphere, and the results can be seen in Fig. 7 below. The results show that the speed of the Moreton wave is about 988 ± 70 km/s.

Fig. 7.The speed of Moreton wave derived from the wavefront position in the chromosphere versus time.

The Moreton wave speed derived from the July 6 phenomenon is not substantially different from that of the June 3 phenomenon. These values are consistent with those previously found for other similar phenomena (Balasubramaniam et al. 2007, 2010; Muhr et al. 2010; Asai et al. 2012).

3.4 EIT Wave of July 6

A similar running difference analysis was also conducted for the EUV images taken by SDO. We also analyzed the 304 Å channel, which can be seen in Fig. 8. From this analysis it was shown that there was no wave observed in EUV channel. Presumably it existed, but because of the projection effect due to the location of the flare, which is near the limb, it was not possible to see it.

Fig. 8.Running difference analysis of EIT images taken in the 304 Å channel. Note that the filament moves upward (arrow), not in the direction of the Moreton wave.

On the other hand, related to the flare, type II and III solar radio bursts occurred right after the flare, as shown in Fig. 9 below. Type III occurred just after the flare, and type II occurred about 5 min later.

Fig. 9.Type II and type III solar radio burst that occured just after the July 6, 2012 flare.

Type II and III solar radio bursts of the X-class flare of July 6 were recorded in Culgoora observatory. The other event is beyond the observation window of this observatory. The type III bursts started to occur at 23:05 UT, while the type II radio bursts started at 23:10 UT. A frequency analysis of the type II bursts yields a rising velocity of about 1,200 km/s.

4. DISCUSSION AND CONCLUSIONS

Although the flares that occured on June 6 and July 3, 2012 both resulted in two kinds of waves, Moreton and EIT waves, these phenomena are very different from each other. The Moreton wave that occurred on June 6 interacted with a coronal hole but there was no corresponding solar radio burst. On the other hand, the phenomenon that occured on July 6, 2012 gave rise to a solar radio burst but there was no interaction with a coronal hole.

Speed analyses from the observed density enhancement in Hα images give the speed of the Moreton wave for the June 3 event as about 926 km/s. The same analysis was conducted for the EIT images from SDO/AIA, in which the cadence is about 12 seconds, and the speed of the shock wave was 1,127 km/s. This resulting speed of the EIT wave exceeds the speed of the Moreton wave. This is not trivial since the common EIT waves have much lower speed. The nature of the observed moving features (both in chromosphere and lower corona layers) is still in question.

For the event of July 6, the obtained speed is 988 km/s and the flare was accompanied by a type III burst that occurred at 23:05 UT, about 4 min late compared to the peak-time of the X-ray flare. However, the radio burst time was still inside the range of the abrupt increase of X-ray intensity. A type II radio burst occurred at 23:10 UT after the ejected material reached a higher and denser region of the corona.

The reflection and transmission phenomena by a coronal hole that occured on the June 3 event proves that the Moreton wave (at least in this event) was a real wave (it obeys transmission and reflection laws). This phenomenon was also discussed by Gopalswamy et al. (2009), and they concluded that this reflection caused some dimming of the wave. A similar result was obtained from our work, with the observation of the deceleration of both the Moreton and EIT waves, as can be seen in Fig. 2. Further evidence that this is a wave can be seen from the filament winking in Hα that occured just after the flare (Fig. 1).

Some interesting events related to these phenomena are the occurrences of coronal mass ejection (CME), which happened just after the occurrence of the flares, where the CME data were obtained from the SOHO LASCO CME catalog (cdaw.gsfc.nasa.gov/CME_list). The CME that occurred on June 3 was a partial halo with a maximum speed of 605 km/s. On the other hand, the CME related with the July 6 event was a halo event with a maximum speed of 1,828 km/s. Wang (2000) argued that the initiation of a CME/flare could trigger an EUV wave. Chen (2016) argued that there are two kinds of EUV waves, fast ones and slow ones, where the fast EUV waves are related with a solar radio burst. With respect to Chen (2016)’s argument, both EUV waves were fast waves, but there are some questions related with the waves that occurred on June 3 because on this day a solar radio burst was not observed.