Ade.SpaceRes.Vol. 11 No.5, pp. (5)191—(5)197,1991 0273—1177/91 $0.00 + .50Printed in GreatBritain. 1991COSPAR

HIGH RESOLUTIONSOLAR PHYSICSFROM ROCKETS

K. P. Dere

E. 0. Hulburt Centerfor SpaceResearch,NavalResearchLaboratory,Code4163, Washington,DC 20375, U.S.A.

ABSTRACT

Explosive events are highly energetic phenomena that are frequently seen throughout thequiet and active sun, particularly in spectral lines formed at transition zone temperatures.Sufficient observational evidence has now been developed to conclusively demonstrate thatsome explosive events are caused by magnetic reconnection in emerging magnetic flux regions.It is also consistent with observations of photospheric magnetic flux cancellation topropose that all explosive events are the result of magnetic reconnection. By combining theobservational facts concerning photospheric flux cancellation and transition zone explosiveevents it can be shown that reconnection in the quiet solar atmosphere proceeds in bursts atsites much smaller than the boundary between opposite polarity flux elements that areobserved to cancel in magnetograph sequences.

INTRODUCTION

The first rocket flight of the Naval Research Laboratory’s High Resolution Telescope andSpectrograph (HRTS) revealed the widespread occurrence of explosive events in the solaratmosphere. These events are characterized by small spatial scales (2”), short time scales(60s), and high velocities (100 kin ~_l) and are most prominent in spectral lines formed attransition zone temperatures (l0~ K) 11,2,3/. It has generally been assumed that theexplosive events consist of plasma accelerated by the action of magnetic forces. Untilrecently, the relevant magnetic field data have generally not been available to test thishypothesis.

Slitless EUV spectroheliograms recorded by the NRL experiment on Skylab/ATM provided some ofthe first evidence of broadened emission line profiles in an emerging flux region /4/. Thebroadening was evident in lines of ions such as Ne VII and Mg IX formed in the uppertransition zone.

HRTS spectra obtained during the Spacelab-2 mission provided the first evidence thatexplosive events seen in C IV are the result of emerging magnetic flux /5/. The C IVprofiles in this explosive event were some of the most spectacular yet seen. Both bulk andrandom motions at velocities up to 300 kin ~l were seen in a 15” by 30” area to thenortheast of the sunspot in AR 4682. A classic arch filament system in Ha indicated thenewly emerging magnetic flux. This was one of the largest and most dynamic explosive eventsobserved and perhaps consisted of a number of explosive events with velocities somewhathigher that usual, but nevertheless, still recognizable as an explosive event.

The second example was reported by Dere et al. /6/. Here, magnetic flux was observed toemerge to the south of the sunspot in AR 4682 on the day previous to that discussed byBrueckner et al. /5,’. The wide profiles are limited to a more compact area and are perhapsconfined to a small loop. The velocities appear to be mostly random and velocities up to 120km s~ are seen. The wide profiles continue over a 13 minute period, with significantfluctuations in intensity and velocity.

In this paper, we present the evidence for our conclusions concerning the identification ofthe sites of the explosive events with the sites of magnetic reconnection. With thisunderstanding, it is then possible to use the observational data relevant to explosiveevents and magnetic flux cancellation to examine the process of magnetic reconnection in thesolar atmosphere.

THE HRTS DATA SETS

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is a stigmatic tandem-Wadsworth mount and records the UV spectrum 1200-1700 A onphotographic film with a spectral resolution of 50 mA. The slit has an equivalent length of1 solar radius and an equivalent width of 0.5. The complete UV spectrum 1200-1700 A can berecorded simultaneously or a wavelength mask can be used to limit the spectral range to aselectable 15 A band to conserve film.

The sixth rocket flight of the HRTS (HRTS-6) took place on November 20, 1988. The main goalof this flight was to observe a coronal hole on the solar disk and an analysis of thecoronal hole data as well as a detailed description of the observing program was presentedby Dere ~ ~. /7/. Briefly, the HRTS slit (900” long by 1” wide) was rastered across theSun in 1” or 2” increments to build up four rasters, three of which are contiguous and passthrough the coronal hole. A total of 97 spectra (exposure time 2.4s) were obtained duringthe operational portion of the rocket flight which lasted from 1834 UT to 1839 UT.

0 The HRTSspectrograph was configured to record a limited wavelength range centered on 1550 A. Theprominent spectral lines of C IV which are formed at a temperature near 1O

5 K are present inthis wavelength range as well as a number ‘of chromospheric lines of C I, Si I and Fe II andthe ultraviolet continuum. Images of the intensity, velocity and line-width of the variouslines were constructed from the wavelength moments of the rastered spectra. In addition,images of the Continuum intensity were constructed in a similar manner. The positions ofthe explosive events were also located on a similar two-dimensional map. A full-diskphotospheric magnetogram was obtained by the National Solar Observatory/Tucson (NSO/T) at1650 UT and a He I Xl0830 spectroheliogram at 2041 UT. Images of the ultraviolet continuum,formed in the temperature minimum region (4 x lO~ K) were used to align the HRTS image tothe photospheric magnetogram. The close association of UV continuum bright points to strongphotospheric magnetic field elements has been demonstrated /8/ and ambiguities in thealignment appear to be quite small.

THE ASSOCIATION OF EXPLOSIVE EVENTS WITH EMERGING MAGNETIC FLUX

Further examples of the association of explosive eventswith emerging magnetic flux wereprovided by the sixth rocket flight of the HRTS (HRTS-6). An emerging active region with asunspot was located within the three contiguous HRTS rasters. This sunspot was firstreported in NOAA AR 5246 at 1226 UT on November 20, 1988, only 6 hours before the RATSflight. Maximum sunspot area was attained sometime between 1550 UT on November 22 and0225 UT on November 23 and the sunspot was last reported on November 25 at 0005 UT /9/. Itseems clear that the HRTS data pertain to an early phase of this active region when magneticflux is still emerging. The NSO/T magnetogram in the vicinity of the emerging active regionis shown in Figure 1. Also shown are the positions of the explosive events identified by

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wide profiles in the C IV lines with maximum velocity greater than 60 km s~ in either thered or blue wing. An unusually high concentration of explosive events is associated withthis emerging active region. This is in contrast to observations of an old active regionobserved with HRTS-3 where very few explosive events were found /3/. This suggests a strongrole for emerging flux in producing explosive events. In the active region itself, thepositions of the explosive events seem to be located along the neutral line separating theopposite magnetic polarities of the strong plage and sunspot fields and also along theoutside boundaries of these fields regions. Outside the active region the explosive eventsare often located just outside the boundaries of strong flux regions.

A second concentration of explosive events apparently associated with emerging flux can beseen in Figure 2 which shows the NSO/T magnetogram and the locations of the explosive

_ ~- -~‘

Figure 2: The NSO/T magnetogram(bright=positive polarity, dark=negativepolarity,gray=negligible field strength) and the positions of the explosive events (X)

in an emerging flux region.

events. The explosive events locations in the center of the figure are arranged along threelines. The magnetogram in this area is relatively featureless. This is in strong contrastto the ultraviolet data. The ultraviolet continuum shows several very bright points in thearea within the lines formed by the explosive event locations. The intensity of the C IVlines in this area is also quite bright but spatially more diffuse than the continuumemission and numerous loops can been seen in the surrounding region. Such loop-likestructures are seen in active regions but it is highly unusual to see such loop structuresin quiet regions. The He I A1O83O spectroheliogram in this area is dark which would beexpected in a region of enhanced coronal EUV emission /10/. The NSO/T magnetogram whichshows only a weak bipolar structure does not present any understandable explanation for theultraviolet observations. The most likely explanation is that a bipolar flux region hasemerged rapidly in the two hours between 1650 UT when the magnetogram was obtained and1834 UT when the HRTS data was obtained and is still present at 2041 UT when the He I Xl0830spectroheliograin was obtained. This example of an emerging flux may be classified as anephemeral region since it did not develop into any long lasting structure. Under thisinterpretation, the explosive events then mark the neutral line between the polarities ofthe new flux and the boundaries between the new flux and the preexisting flux, some of whichwill merge with the new flux and some will cancel. denending on their relative polarities.

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have been particularly useful in determining the location of the explosive events inrelation to the photospheric field structure. The HRTS-5 data were obtained on December 11,1987 within one-half hour of full disk X-ray images recorded by the AS&E rocket payload.Nearly simultaneous data also included the NSO/T magnetogram and He I Xl0830spectroheliogram. An analysis of the combined data set shows that there is a generaltendency of the explosive events to occur on the edge of the C IV network and to avoidstrong field regions /11/. Another recent study using the Spacelab 2 HRTS data and theNSO/T magnetogram has also demonstrated that in the quiet sun, explosive events are wellcorrelated with the network magnetic fields but with a tendency to occur on the edge of thenetwork /12/. Quiet regions observed in the HRTS-6 data show a similar relationship betweenthe photospheric magnetic fields and the explosive event locations. This relationship isillustrated in Figure 3 where the photospheric magnetogram and the corresponding explosive

Figure 3: The NSO/T magnetogram (bright=positive polarity, dark=negative polarity,gray~negligible field strength) and the positions of the explosive events (X)in an a area of the quiet sun.

event locations are shown. These observations were obtained on the same latitude as thecoronal hole observed with HRTS-6 and consequently are not strictly representative of thequiet sun. The chromospheric network is clearly visible in the magnetogram although thenetwork elements are predominantly of a single polarity. Clearly, the explosive events seenin this figure avoid the high field areas and occur mainly on the edges of the strong fieldareas, the network, at places where the observed field is negligible. Analyses of theSpacelab 2 observations /12/, the HRTS-5 observations /11/, and the HRTS-6 observations

(presented here) all lead to the consistent conclusion that explosive events are associatedwith the network fields but that there is a striking tendency for them to occur on the edgesof the network field regions.

EXPLOSIVE EVENTS AS THE RESULT OF MAGNETIC RECONNECTION

The previous observations cited here together with the new data presented here, lead us toconclude that some explosive events are directly associated with emerging magnetic flux.With the emergence of new magnetic flux into preexisting field patterns, magneticreconnection is almost certain to proceed and it is this process that produces theacceleration of the bulk plasma that is the observed signature of explosive events. We makethe further proposal that all explosive events are the direct result of magneticreconnection in the solar atmosphere. The primary evidence for concluding that magnetic

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fields in the quiet sun has been constructed /13,14/. New magnetic field typically emergesat the supergranular network cell centers as small bipoles with field strengths considerablysmaller than in the network. Often these fields are separated and convected to the networkboundaries where they either merge with network fields of the same polarity or cancel withfields of the opposite polarity. This cancellation occurs in the same general locationwhere explosive events tend to be found. Similar cancellations in active regions result insolar flares /15,16/. We identify the process of cancellation as the magnetic fieldreconnection which accelerates plasma to produce the explosive events observed. Thisprocess is illustrated schematically in Figure 4. Another factor which makes this

\ !~ I SUPERGRANULAR\ ‘~ I CELL BOUNDARY

EMERGING INTRANETWORK FLUX\\ISUPERORANULAR CELL BOUNDARY

\ \ \\ \ ~ (I// RECONNECTION SITE\ \~~7/~f/~”INTRANETWORK FLUX

Figure 4: A schematic representation of intranetwork magnetic flux emergence andreconnection.

identification plausible is the bipolar nature of explosive events. Strong emissions aretypically seen in both the red and blue wing or sometimes in only one wing of transitionzone lines. Sometimes the spatial locations of the two wings are slightly different. Thissuggests a reconnection model which accelerates plasma both upward and downward but atdifferent places.

The general class of rapid, small-scale brightenings of transition region and coronal linesin active and quiet regions has been addressed previously by a number of authors but onlythe study of Porter et al. /17/ has examined the relationship of these brightenings to thephotospheric magnetic field patterns with any definite result. They found that essentiallyall of their short-lived, as well as the long-lived, brightenings were associated withmagnetic bipoles and suggested that they were most likely identical to the explosive eventsobserved with the HRTS. Moses ~ a].. /11/ showed that in the HRTS-5/AS&E data set someexplosive events were to be identified with coronal bright points and bipolar magnetic fieldregions but that this was not the general case. The difference between the results ofPorter ~ al. /17/ and the present results is probably due to the fact that different kindsof events have been analyzed. The explosive events have been selected because of their highvelocities and most are not especially bright whereas the events of Porter ~ ~1. wereselected because of their brightness.

DIAGNOSTICS OF MAGNETICRECONNECTIONIN TUE SOLARATMOSPHERE

With the understanding that explosive events and flux cancellation are manifestations of theprocess of magnetic reconnection, it is possible to use the observed characteristics ofthese phenomena to probe the nature of magnetic reconnection as it occurs in the quiet solar

(5)196 K. P. Dere

boundary between the opposite polarities that is typically 5,000—10,000 km in length. Theaverage size of an explosive event is 1,500 km and, except for the case of the emerging fluxregions shows in Figures 1 and 2, snapshot images of the sites of explosive events do notgenerally delineate a continuous boundary of some 5,000 - 10,000 km suggested by themagnetograph images. Times scales for flux cancellation are on the order of 1-5 hours andthe average lifetime of an explosive event is 60 s but many show lifetimes at the HRTS-3raster cadence of 20 s. The conclusion to be drawn is that reconnection occurs in a burstymanner at intermittent locations along the boundary separating the opposite polarityelements that are cancelling and reconnecting.

SUMMARY

These observations lead us to conclude that in many cases explosive events are directlyrelated to emerging magnetic flux where they are almost certainly the result of magneticreconnection. Further, we propose that all explosive events are caused by magneticreconnection. This proposal is based on the identification of the sites of the explosiveevents with the location of magnetic flux cancellation often observed by the Big Bear SolarObservatory at the edges of the quiet solar network 113/. One of the main goals of the nextHRTS rocket flight will be to demonstrate the identification of explosive events withcancelling photospheric magnetic flux.

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