A study of the unification of quiet-Sun transient-event phenomena RA Harrison, LK Harra, A Brkovic, CE Parnell (2003), A&A, 409, 755 David H. Brooks, Short talk, Solar Seminar, Kwasan Observatory, Kyoto University 9月13, 15:00

　　　　　 Background Ubiquitous transient events have been observed occurring over 　the entire solar disc, e.g., blinkers, explosive events, EUV network 　&　cell brightenings, network flares (Yohkoh), nanoflares etc. 　These events may provide direct evidence of processes such as 　magnetic reconnection, plasma acceleration and heating. 　Therefore understanding them may help us to understand coronal heating, solar wind acceleration etc. A complex picture has emerged of these events. However, this is at least partly because event type classifications are driven to a large extent by the instrumental limitations and different observing techniques used, rather than by different physical processes!

Some fundamental questions Are all these event types simply the same phenomenon? Do they just appear different because of the different ways they are observed? If they are different phenomena, are we able to distinguish between fundamentally different event types and perhaps unify some of the different classes?

Objectives of this paper Review the different types of quiet-Sun transient events to try to confirm or deny that they are the same. Review the observational results, obtained by different instruments and techniques, and try to identify any unification between classes of events. That is, take into consideration how the different instruments’ observations SHOULD differ. (These authors have studied quiet Sun blinkers (Harrison), EIT brightenings (Brkovic), active region blinkers (Parnell), cell and network brightenings (Harra) using different instruments/methods, so they have tried to extrapolate between ALL their results. Not a modeling paper: it demonstrates that operational and instrumental differences have contributed significantly to the way a number of small-scale phenomena are classified, when they are most probably the same.

EUV brightenings at transition region temperatures BLINKERS: Small brightenings in the EUV. Harrison (1997) & Harrison et al. (1999) studied many blinkers visually, using SOHO/CDS, and identified them as intensity increases (about 60%) primarily in transition region lines of e.g. OIII, OIV, OV (characteristic temperatures 100-250,000K). Fewer events in chromosphere/ corona. Durations 400-1600s. Global birth rate 1.24 per second I.e. 3000 on the Sun’s surface at any moment. Predominantly in the network. Due to density or filling factor increase (emission line ratios are flat I.e. no temperature change. 100’’

EUV brightenings at transition region temperatures Bewsher et al. (2002) developed an automatic detection routine to allow them to study a far greater number of events. They searched for peaks of 9-30 sigma in OV (MUCH MORE SIGNIFICANT PEAKS THAN FOR NANOFLARES). Fewer events of such size seen in chromosphere (He I), corona (Mg IX, X): 25% of OV blinkers seen in He I (20,000K) with peaks of 15 sigma, 50% with peaks 9 sigma, 90% with peaks 3 sigma. 8% & 13% of OV blinkers seen in Mg IX & Mg X with peaks of 3 sigma. Parnell (2002) studied active region blinkers and found similar results: slightly larger intensity increases and higher frequency. 100’’

EUV brightenings at transition region temperatures Therefore, these authors consider the properties of blinkers to be: GLOBAL FREQUENCY: 10-20 per second MEAN INTENSITY INCREASE: 70-80% MEAN AREAS: 2-3 X 107 km2 (6-7.5 arcsec2) MEAN LIFETIMES: 16.4 minutes Bewsher et al. (2003) also showed that blinkers occur anywhere (network, cell centres etc.). Parnell et al. (2002) found them above plage regions and the umbra and penumbra of sunspots. 100’’

EUV brightenings at TR temperatures All these studies used CDS, but CDS is a scanning spectrometer so the time cadence is low (in these studies the best was 150s). Berghmans et al. (1998) used EIT and found many EUV brightenings in He II (80,000K) using a higher cadence of 1 minute. Are they blinkers? The global birth rate was found to be 15-20 per second for 2.5-3 sigma increases. Given that 90% of OV blinkers detected with CDS were seen in He I, these results are consistent. He II events had durations 2-20 mins. Given CDS cutoff at high cadence, these results are identical. Areas for the EIT events were 1-10x107 km2 and intensity increases were 30-100%, again consistent with CDS results. Events seen in network and cell interiors, contrary to Harrison et al. but in agreement with more extensive study of Bewsher et al. Blinkers, and EIT He II brightenings are the same event type! 100’’

EUV brightenings at TR temperatures Similar events have been discussed by Gallagher et al. (1999) and Harra et al. (2000), namely, cell & network brightenings. These are EUV brightenings observed by CDS and given different names because blinkers were initially thought to occur only in the network. However, as we know, they are also found in cell interiors, though the network is more common. Harra et al. (2000) studied 1125 OV events using ‘sit and stare’ observations with a narrow slit rather than scanning. Therefore, they could not discuss the sizes of the events, but they found the following results: GLOBAL FREQUENCY: 2177 per s (network) 2635 per s (cell) MEAN OV INCREASE: 10% MEAN LIFETIME: 150 s (network) 96s (cell) Many more low intensity short duration events! 100’’

EUV brightenings at TR temperatures Are they the same as blinkers? A third technique allows us to cross-check. Harra et al. & Brkovic et al. used two very different observing techniques with CDS, e.g. Harra et al. Brkovic et al. 4” x 240” slit 90” x 240” slit (no spectra) 15s exposures 31s cadence Sun rotated through the image. Feature tracking on. Brkovic et al. therefore should be able to also detect shorter duration events. They found : GLOBAL FREQUENCY: 22.1 per s MEAN INTENSITY INCREASE: 65% MEAN AREAS: 2.2x107 km2 MEAN LIFETIME: 15 mins. These results agree with Bewsher et al. not Harra et al. *Since Harra et al. have spectra, they can subtract the background. 100’’

EUV brightenings at TR temperatures Therefore, are Harra et al. seeing something different? Harrison et al. checked by taking a single pixel column from Brkovic et al.’s data and applied both analysis techniques to the same data. 100’’ EM Slit smooth network visible, boxes background subtracted showing show events detected by Brkovic. events detected by Harra et al.

EUV brightenings at TR temperatures From the Brkovic et al. results we would expect about 26 events in these data. In fact Harrison et al. detected 30 and they show durations of around 5-30 minutes with 45% intensity increases. However, Harra et al. detected many more events of short duration and intensity increases of 10% or so. Therefore, this demonstrates that the differences in the results of Harra et al. & Brkovic et al. are mainly due to the observing sequences used, and the data analysis techniques employed. The rapid single slit images, slower scanning, and wide slit snapshot observations are showing the same events, but characterising them differently. Blinkers, EUV brightenings, cell interior & network brightenings are all the same event type! 100’’

Explosive events and blinkers Explosive events do not have a strict definition, they are small-scale (1500-4000km) and show high velocities (100-150 km/s). They are seen in both network and cell regions (like blinkers): GLOBAL FREQUENCY: 500 per second AREAS: 2.3x106km2 MEAN LIFETIMES: 60s 50% of EEs appear associated with EUV brightenings seen by CDS. However, SOHO/SUMER observations are limited by single narrow slit locations. The two instruments limit our ability to link these two event types. NB, CDS spatial resolution 4”x1.7”, SUMER spatial resolution 2”x2”  SUMER covers 60% of CDS slit. If only 10% of SUMER pixel area shows motion, then only 6% of CDS does!

Explosive events and blinkers However, several papers have attempted to study their relationship: Chae et al. (2000) found that EEs occur on the edges of OV brightenings observed by CDS. Innes (2001) showed that CIV TRACE images showed brightenings in association with SUMER velocity events. BUT, neither study confirmed that the EUV brightenings have the same characteristics as blinkers. NB, CIV TRACE images are broadband filter images so contain emission from plasma at other temperatures. Bewsher (2002) re-examined the data of Innes (2001) and compared the EEs with blinkers she detected automatically in OV. She found 1 EE at the border of a blinker (which did not show any enhanced wings - predicted for high velocities), 7 EEs not connected to blinkers, and many blinkers with no EEs.

Explosive events and blinkers In all cases the blinkers and EEs occurred in the network, possibly giving a misleading impression that they have some connection. Madjarska & Doyle (2003) obtained joint CDS/SUMER observations. They identified OV blinkers by eye and then compared with simultaneous Ne V data from SUMER. They identified 3 blinkers in OV and Ne V, and also 2 EEs in Ne V. The EEs occurred on the edges of the OV blinkers and they did not seem to correspond to any particular behaviour of the blinkers (line profiles or otherwise). They conclude that blinkers and EEs are not intimately related.

Explosive events and blinkers That they are probably not related need not be a surprise, blinkers appear to be density/filling factor events (not temperature events), whereas explosive events show high velocities and could well be related to reconnection. On the other hand, if they are related, then any theory needs to explain: why are there so many more EEs than blinkers? why do EEs have so much shorter lifetimes? why do the Ne V blinkers not show high velocities?

Transient events in the corona Harrison et al. (1999) reported only small effects in the corona during a blinker (or no effect at all). E.g. 4-7% in lines of Mg IX and Mg X. Bewsher et al. (2000) found NO brightenings in coronal lines (remember they were looking for strong events) BUT 7-13% of OV blinkers showed weak (20% increases) in Mg IX and Mg X. HOWEVER, these authors did not search for coronal brightenings independently I.e. they looked for OV blinkers and then checked if they registered at higher temperatures. Brkovic (2001) DID search independently and found about 6 times more Mg IX brightenings than OV. Only 20% coincided with OV. 1) single event type but usually located at one temperature (TR or corona) and rarely extending between the two, OR, 2) two types, one restricted to one region, the other a heating/ cooling event viewed at different temperatures.

Transient events in the corona Other authors have discussed transient events in the corona using e.g. EIT, TRACE and different observing techniques: EIT network flare & heating events (Krucker & co-workers, 97,98) TRACE nano-flares (Parnell & Jupp 2000, Aschwanden et al. 2000) These are generally accepted to be the same because the energies are essentially the same, e.g. 1023-1026 ergs. Also, their sizes (3x107km2) and durations (10 minutes) are similar to blinkers. Again, do they have something in common with blinkers? The birth rate of the heating events is 0.33 per second, much smaller than that for blinkers. However, instrumental effects could again be important. Harrison et al. checked how many “heating events” would be expected in the CDS data of Bewsher et al. They calculated that 6 events should be found, but in fact about 30 were found!  lack of coronal signature in blinkers is real.

Transient events in the corona Benz & Krucker (1998) extended this work to higher temperatures, finding a higher birth rate (293 per second) for 1 MK brightenings in Fe IX/X, and XII. Parnell & Jupp (2000) used TRACE data of the Fe IX/X 171 A line and found results implying a birth rate of 92-727 per second. Aschwanden & Parnell (2002) also looked at TRACE Fe IX/X data and found a birth rate close to that of blinkers (23 per sec). They chose only “flare-like” cooling events so these should be observed with CDS in Mg IX, X. Why did Bewsher et al. not detect many events in Mg IX/X, the formation temperature is similar to Fe IX/X? Probably, because the cadence was too low, they were searching for large events (coronal brightenings seem to be smaller) and they did not search independently via Mg IX.  Someone should check this….

Conclusions: transition region Taking into account the differences in instrumental characteristics and observing sequences, it was found that, blinkers, EUV brightenings, cell interior and network brightenings are most probably all the same event type. A review of simultaneous UV and EUV observations suggests that there is little evidence that blinkers and explosive events are directly related.  there are at least two types of transition region phenomena! magnetic reconnection driven explosive events with 100-200 km/s outflow jets or shocks. density/filling factor events with low/no velocities caused by a compression mechanism e.g. merging of magnetic fields.

Conclusions: corona The results for coronal brightenings are less clear. However, Only 20% or so of the OV blinkers show 1MK counterparts. A consideration of the > 1MK brightenings show that although some are associated with blinkers, the majority show no TR signature.  inconclusive whether blinkers are the same as the coronal events, but at least SOME are related (unlike most EE and blinkers) Possibilities: blinkers and nano-flare/heating events created by the same mechanism but mainly confined to a limited temperature range, there are two types of blinker/high temperature phenomena: a) the compression event driven my increased filling factor, b) mini flare-like which reaches high Te (reconnection driven), Explosive event or high temperature event may just depend where reconnection takes place (chromosphere or corona).

My comments Brooks & Kurokawa (2004), ApJ, 611, 1125, found more weak Mg IX events than OV events (3473 compared to 3302) in their high cadence data. They also found 20% of OV blinkers register in Mg IX, in agreement with Bewsher et al. (2002) (CDS) and Brkovic (2001) (EIT). This solves this discrepancy! Chae et al. (2000) and Brooks et al. (2004), ApJ, 602, 1051, showed that blinkers are actually composed of small-scale (2-3”), short duration (2-3 minute) impulsive bursts, similar in size and time- scale to explosive events.  the properties are slightly different to those that have been described, Brooks & Kurokawa (2004) reviewed their properties and they are similar to those Harra et al. (2000) found. Bewsher et al. (2003), Madjarska & Doyle (2003), Brooks et al. (2004), found that many blinkers are red-shifted and show line broadening, suggestive of some dynamical characteristics (albeit at a lower level).

Chae et al. (2000) proposed an explanation Chae et al. (2000), proposed that explosive events and blinkers are caused by different magnetic reconnection geometries. Bidirectional flows are produced which appear as broadened profiles. Not LOS for blinkers. 100’’ Explosive event Blinker