Astrophysics Division, Rutherford Appleton Laboratory, Chilton, Didcot,
Oxfordshire OX11 0QX, UK,
In Spring 1994 the Inter Agency Consultative Group Campaign III was set up at a kick-off meeting in Sagamihara, Japan. The basic idea of Campaign III was to coordinate solar, interplanetary and near-Earth spacecraft in an effort to link solar and near-Earth observations relating to activity such as solar mass ejections, and features such as co- rotating interaction regions. The Campaign was split into two basic activities, namely (i) studies of the onset of mass ejections, and (ii) studies of the effects of mass ejections and other solar phenomena at the Earth. The onset of the Campaign activities was effectively linked to the launch date of the Solar and Heliospheric Observatory (SOHO) spacecraft, since most of the planned activities centred on use of SOHO data linked to other spacecraft. SOHO was launched in December 1995 and the onset of full scientific operations was in March/April 1996. At the time of writing the more complex, time consuming multiple instrument operations such as the ones required for the IACG campaign have not been run, though pilot studies between selected instruments have been performed. Thus, we have preliminary observations relevant to the Campaign, and we anticipate the full campaign tobe run on several occasions before the end of the year. In this paper, we report on the IACG Campaign III concept and on the preliminary results from the pilot studies.
Given the availability of a large international fleet of solar, heliospheric and near-Earth spacecraft, it is imperative that coordinated observations and analyses are arranged, in order to best determine the relationship between the Sun and the Earth-system. Thus, the international body which guides multispacecraft operations, the Inter Agency Consulatative Group (IACG) met in Sagamihara, Japan on 31 May to 2 June 1994 to discuss the IACG Campaign III, Solar Events and Their Manifestation in Interplanetary Space and in Geospace. A full report on this meeting was produced by Domingo and Fleck (1995).
The Sagamihara meeting came up with a three pronged strategy for IACG Campaign III in the form of three sub- campaigns. These were:
The practical application of these sub-campaigns was to be left up to sub-campaign chairmen under the leadership of the author. Indeed, it was quickly decided that the CME onset sub-campaign should be directed by the author, and the CME propagation sub-campaign should be directed by Dr Dan Baker (Boulder). A subsequent report on the plans for the CME onset sub-campaign was published by Harrison (1994) and details of both sub-campaign plans are displayed on the World Wide Web (URL: http://sohowww.nascom.nasa.gov/iacg3/).
The `quiet Sun' sub-campaign proved to be more difficult to implement mainly because it was far less focussed. In addition, the IACG Campaign IV, entitled Solar Sources of Heliospheric Structure Observed out of the Ecliptic, seemed to have a large overlap with this topic. The IACG Campaign IV topics are specifically listed as "large scale heliosphere and its dependence on solar photospheric and coronal phenomena","topology of coronal steamers", "three dimensional shape of coronal hole boundaries" and "solar wind source and evolution". In response to this, the three part effort for IACG Campaign III has been modified to focus more on CMEs with the second and third topics taking the most effort. Topics relating to quiet Sun structure may be covered in the third sub-campaign as necessary, and it has been recognised that a close collaboration is required in this area between the IACG Campaigns III and IV.
The two remaining sub-campaigns, which can be listed, roughly, as CME onset studies and the propagation and effects of CMEs are very different in nature. The former requires the design of multi-instrument observations looking at potential CME sources on the Sun with supporting interplanetary instrumentation. The latter can be done more in hindsight, i.e. when we have CME event lists, we can locate complimentary data-sets from an array of spacecraft and study the effects in the heliosphere and magnetosphere. It was clear from the earliest discussions of Campaign III that the keystone of the study would be the ESA/NASA Solar and Heliospheric Obervatory (SOHO) spacecraft, providing coronagraph data for the recognition of CMEs and extreme ultraviolet and ultraviolet imagers and spectrometers for the detailed observation of CME source regions, providing diagnostic information such as densities, temperatures, flow velocities and abundances, as a function of time.
SOHO was launched on December 2, 1995. It took 2.5 months to travel to its operating position at the Sun-Earth L1 Lagrangian point. During this time the spacecraft and the instruments were commissioned. The payload moved into a fully operational scientific programme in March/April 1996. Details of the SOHO payload can be found in the December 1995 issue of Solar Physics and are summarised by Domingo et al. (1995) in the same issue.
THE OBSERVING PLAN
The foundation for Campaign III is the setting up of a good observing
scenario for SOHO. The basic design for this was made in 1994, and reported
at the SOHO Workshop in Estes Park in September 1994 (Harrison, 1994).
The design has been modified as a result of SOHO first light observations
but the basic technique is the same, and can be summarised as:
For details of the SOHO instruments listed, the reader is referred to issue 162 of Solar Physics. However, the above SOHO observing sequence goes under the name of JOP 3 - Joint Observing Programme 3.
This basic package of observations defines the principal tool for the detection and investigation of CME onsets. Onto this we build the multi-spacecraft and ground based observations necessary to track CMEs and investigate their effects. For the observation of the eruption and associated solar activity, this would include a selection of the following: (i) For coronal and large-scale `context' observations, we require coincident observations from the Yohkoh Soft X-ray Telescope, which may operate in its standard synoptic mode of operation in support of this activity (contact: Dr T. Watanabe). (ii) For CME detection and tracking, to confirm and extend the SOHO coronagraph observations, we require supporting data from the High Altitude Observatory's Mk III k-coronameter on Mauna Loa Hawaii (contact: Dr D. Hassler). (iii) Supporting prominence observations are required and may be obtained from several sites, e.g. the Mauna Loa Prominence Monitor, the Wroclaw Observatory, Big Bear and the SOON network (contacts: Dr A. Kiplinger and Dr D Hassler).
Moving on to considering the effects of CMEs in space and in geospace, there are many available spacecraft. SOHO carries a package of in-situ detectors (see issue 162 of Solar Physics) in case we see Earth-directed CMEs. Suitable instruments are also aboad Ulysses, Polar, Wind and Interball and contacts must be made with these groups as CMEs are detected during Campaign III activities. Much of the work in this area can be done after the event, unlike the CME onset detection process.
Returning to the operation of SOHO, general operational issues are guided by a Science Working Team (SWT, i.e. the 12 Principal Investigators) and major inter-instrument operations such as that required for the IACG campaigns have to be agreed at the SWT meetings for implementation in the following months. To date, the SWT has not defined dates for the first `official' runs of JOP 3. This is principally because the CME rate has been reported to be very low and the feeling is that in order to persuade the pointed instruments to sit on selected targets for several days, we should be able to argue that there is a reasonable chance of an ejection. However, the definition of a `reasonable chance' and of the CME rate where such an observation becomes viable is clearly a matter of individual opinion.
The majority of the SOHO observing activities to run JOP 3 have been run already, by the individual instruments, to check their operation. For example, the CDS 4x4 arcminute raster designed for this operation has been run on a number of targets, and the LASCO and EIT sequences are basically part of well-tested synoptic activities.
Even though JOP 3 has not been run, we do have new SOHO data relevant to the observation of CME onsets, which may provide a significant step forward in our understanding of CMEs. Preliminary event lists have been compiled by members of the LASCO team and it turns out that quite by chance, some of the CME source regions were viewed by the `surface' instruments. This will be discussed more in the next section. In addition it is noted that as the SOHO mission develops, and as people become more familiar with their instruments, the routine checking of new data-sets for the identification of CMEs will become easier. Indeed, recent event lists are suggesting that there have been several tens of events seen by the SOHO coronagraph to date. With this in mind, maybe it is time to press for the JOP 3 observations to be run.
PILOT RUNS To discuss pilot studies of Campaign III, we concentrate of two instruments from the SOHO payload, namely the LASCO coronagraph (Brueckner et al. 1995) and the CDS spectrometer (Harrison et al. 1995). LASCO allows the detection of CMEs and the identification of geometry, speed and structure. CDS provides detailed extreme ultraviolet spectra which may provide diagnostic information on the source region. CDS can be used to image 4x4 arcminute portions of the Sun, without repointing, in a number of selected spectral emission lines. The choice of emission lines allows a selection of desired temperatures and density diagnostics to be viewed. CDS is particularly important for this kind of work because it views plasmas in the temperature range from 20,000 to 2,000,000 K.
As mentioned in the last section, we have already made several observations where the CDS instrument was making large-scale images below a region of the corona where LASCO detected a CME. Thus, we already have a few studies which can be used as pilot studies to the running of JOP 3. The success of these will, no doubt, boost the support for running JOP 3 with the full SOHO compliment.
Scanning an individually compiled LASCO event list (St Cyr, June 1996, private communication) and comparing it to the CDS activity catalogue, for the period up to June 22nd, we have found several observations with consistent data-sets from the two instruments. Some of these joint events are listed in Table 1. This is not a complete list, and should be regarded with caution prior to the release of an official event listing from the LASCO team. The CDS observations were all made with a series of emission lines in a temperature range from 20,000 to 1,000,000 K or higher.
Table 1. Some coincident LASCO-CDS observations - An IACG Campaign 3 pilot study event list
|LASCO `Event'||CDS Observations|
|March 22 1996 - Event in NW, 03:42-19:44? UT, `Material ejected along streamer'||4x4 arcmin images of filament on NW limb 03:31-07:29 UT.|
|May 1 1996 - Event in E, 03:00-07:00 UT
`Giant structured loop'
|2x4 arcmin images of prominence on SE limb 07:01-11:04 UT. (post event?)|
|June 16 1996 - Main event in E, but activity in W. from >14:50 UT||2x2 arcmin images of loop system on W limb 15:31 - 19:47 UT|
|June 18 1996 - Faint arc in SW, early on 18th.||Single 4x4 arcmin image of loop system at 20:43 on 17th, on SW limb|
|June 19 1996 - Event in E, From about 12:00 UT||Series of 4x4 arcmin and 2x2 arcmin images of E limb loop system at 06:26-08:34 UT (Pre-event?)|
|June 22 1996 - Event in E, from ~00:30 UT||`Evolving prominence' seen in NE in 2x4 arcmin images 20:53 - 22:41 UT on the 21st.|
We discuss now some of the events listed in Table 1 in greater detail, illustrating the CDS observations for two of them. This demonstrates well the power of the CDS instrument in particular, with its ability to select the required temperature regimes of the solar atmosphere at the CME source regions. All of the events listed require further analysis to ensure that any activities are related and to investigate the processes at work, but the point we wish to stress here is that the data here show that we have the potential for unprecedented observations of CME onsets.
For the event on March 22, 1996, the CDS instrument was used to observe a filament as it approached the north west limb, over a 4 hour period. Shortly after the start of this period LASCO observers reported material being ejected along a streamer in the north west, lasting for many hours. The CDS observations are illustrated in Figure 1, with a selection of three 4x4 arcminute observations, taken at 03:31, 05:47 and 07:29 UT. They are part of a series of eight images taken, at a cadence of 34 minutes. For each image, intensities were recorded in a set of eleven carefully selected emission lines in the extreme ultraviolet. In effect, these allow us to view different temperature regimes from 20,000 K to 2,000,000 K. For Figure 1, we show images from three emission lines, namely, neutral helium (He I 584 Å at 20,000 K), and highly ionised oxygen (O V 629 Å at 250,000 K) and magnesium (Mg IX 368 Å at 1,000,000 K), from left to right. These illustrate how CDS can take the Sun's low corona and transition region to pieces through careful line selection.
The helium and oxygen images are representative of chromospheric and transition region temperatures. They show the supergranular network but, more important for this work, they show a filament, seen in absorption approaching the north west limb. Little is detected in the corona. The magnesium image also shows the location of the filament, through absorption, as well as the bright corona, but few other surface features. We are primarily concerned with the prominence and any coronal activity in this work; for further discussion of features seen by CDS the reader is referred to Harrison et al. (1996a,b).
By 05:47 UT we can see clear evidence for material extending above the limb, particularly in the oxygen emission. However, the bulk of the filament structure appears to be unchanged. Again, by 07:29 UT, we see evidence for further changes, with structures extending into the corona, but the filament itself has remained. One striking feature of the 07:29 UT images is the `S' shaped feature on the limb to the south of the filament. Note that during this entire period, the preliminary LASCO event listing reports material being ejected through an overlying streamer in the north-west.
Further analysis of these data are required to assess what is happening,
and this is beyond the scope of this paper, but it seems that this event
has been `caught' both in the high corona, and in the low corona and transition
region. It is a good example of how we can use the instruments on SOHO
together, to attack specific questions.
Fig. 1 CDS observations of March 22, 1996. A 4x4 arcminute area is shown for images taken at 03:31 (top row), 05:47 (middle row) and 07:29 UT (bottom row). Each row contains images taken simultaneously in He I, O V and Mg IX light (see text).
For the May 1 event, LASCO saw a giant ejection on the eastern limb at the same time as observing the comet Hyakutake above the northern pole. CDS was producing images of a prominence on the south eastern limb towards the end of the period of the reported eruption but saw evidence for post event activity with large-scale loop structures, particularly in the cooler emissions from helium and oxygen ions. The location is most likely consistent with the southern footpoint of the LASCO event, but the precise coalignment needs to be checked.
Fig. 2 CDS observations of June 21, 1996. A 2x4 arcminute area is shown for images taken at 20:53 (top row), and 22:41 UT (bottom row). Each row contains images taken simultaneously in He I, O V, Mg IX and Si XII light (see text).
For the June 22 event the LASCO team simply note an event in the east at the start of the day. Shortly before this, CDS saw significant changes in a prominence, showing structure which seemed to disappear (erupt?) between 20:53 and 22:41 UT on the 21st. The event is illustrated using two sets of CDS images taken at 20:53 UT and 22:41 UT shown in Figure 2 (top and bottom, respectively). The images show, for each observation, emission from neutral helium (He I 584 Å), and highly ionised oxygen (O V 629 Å), magnesium (Mg IX 368 Å) and silicon (Si XII 520 Å). These are representative of temperatures of 20,000 K, 250,000 K, 1,000,000 K and 2,000,000 K and thus show a good cross section of the atmosphere. Many more emission lines were recorded for this event; we choose to show only four. In addition, we choose to show images from only two sets of observations; a further observation was made at 21:47 UT.
The CDS images show a region on the solar limb approximately 2x4 arcminutes in size. A prominence can be seen in the helium and oxygen emission. In the million degree magnesium image the prominence can be seen, but only because it occults coronal emission from behind. Finally, at 2 million degrees, one sees very little. Again, for further discussion of features seen by CDS, one should refer to Harrison et al. (1996a,b).
By 22:41 UT one sees significant changes. The prominence has changed, with an extension towards the top of the earlier images being absent. However, the major component of the prominence is still intact, and there is no evidence for change in the hotter emissions from magnesium and silicon. These changes, recorded in the CDS image taken at 22:41 UT were followed by the noting of a coronal event in the LASCO data from approximately 00:30 the following morning on the east limb.
Again, further analysis is required to assess the relationship between the LASCO and CDS events, but significant coronal changes were detected by CDS shortly before a coronal event was recorded by the LASCO over the limb being observed by CDS. This coincident observation, along with the multi-wavelength imaging of CDS, again, illustrates the power of these observations.
The mechanics of the SOHO JOP 3 which form the backbone of the IACG Campaign III are in place and, for some instruments have been tested. The JOP has not been run `officially' to date because of the low CME rate, though several consistent data-sets between, in particular, the CDS spectrometer and the LASCO coronagraph have been identified. These are being used not only to start serious studies of CME onsets with SOHO, but as pilot studies to the running of the full campaign. It is in this spirit that they are presented in this report, and it is felt that with the success of these, the full IACG Campaign III CME onset observation programme should begin in earnest.
It is important to realise that SOHO scientific operations started as recently as March/April 1996, and given such a complex mission, we are still climbing the learning curve for making the most efficient, best coordinated observations. We feel that the time is now right to start more complex operations, such as JOP 3, and it is important to start this while we still have a fully operational Yohkoh payload.
Note added after submission: The JOP 3 campaign was selected by the SOHO Science Working Team for running on several occasions in the period October-December 1996. The first JOP 3 run started in the week of October 7.
Brueckner, G.E. and 14 co-authors, The Large Angle Spectroscopic Coronagraph (LASCO), Solar Phys., 162, 357 (1995)
Domingo, V. and B. Fleck, Toward a New Era of Global Solar-Terrestrial Research - Report of the Workshop held in Sagamihara, Japan, 31 May to 2 June 1994, ESA Space Science Department (1995)
Domingo, V., B. Fleck and A.I. Poland, The SOHO Mission: An Overview, Solar Phys. 162, 1 (1995)
Harrison, R.A., The Inter Agency Consultative Group Campaign to Study Coronal Mass Ejections, Proc. Third SOHO Workshop, ESA SP-373 (1994).
Harrison, R.A., E.C. Sawyer and 37 co-authors, The Coronal Diagnostic Spectrometer for the Solar and Heliospheric Observatory, Solar Phys., 162, 233 (1995)
Harrison, R.A., A. Fludra, C.D. Pike, J. Payne, W.T. Thompson, A.I. Poland, E.R. Breeveld, A.A. Breeveld, J.L. Culhane, O. Kjeldseth-Moe, M.C.E. Huber. and B. Aschenbach, High Resolution Observations of the Extreme Ultraviolet Sun, Solar Phys. submitted (1996a)
Harrison, R.A., A. Fludra, E.C. Sawyer, J.L. Culhane, K. Norman, A.I. Poland, W.T. Thompson, O. Kjeldseth- Moe, B. Aschenbach, M.C.E. Huber, A.H. Gabriel and H.E. Mason, Extreme Ultraviolet Observations of the Solar Corona: First Results from the Coronal Diagnostic Spectrometer on SOHO, Adv Space Res. submitted (1996b)