M. L. Kaiser

WAVES: The radio and plasma wave investigation on the wind spacecraft

The WAVES investigation on the WIND spacecraft will provide comprehensive measurements of the radio and plasma wave phenomena which occur in Geospace. Analyses of these measurements, in coordination with the other onboard plasma, energetic particles, and field measurements will help us understand the kinetic processes that are important in the solar wind and in key boundary regions of the Geospace. These processes are then to be interpreted in conjunction with results from the other ISTP spacecraft in order to discern the measurements and parameters for mass, momentum, and energy flow throughout geospace. This investigation will also contribute to observations of radio waves emitted in regions where the solar wind is accelerated. The WAVES investigation comprises several innovations in this kind of instrumentation: among which the first use, to our knowledge, of neural networks in real-time on board a scientific spacecraft to analyze data and command observation modes, and the first use of a wavelet transform-like analysis in real time to perform a spectral analysis of a broad band signal.

Interplanetary acceleration of coronal mass ejections

Using an observed relation between speeds of CMEs near the Sun and in the solar wind, we determine an “effective” acceleration acting on the CMEs. We found a linear relation between this effective acceleration and the initial speed of the CMEs. The acceleration is similar to that of the slow solar wind in magnitude. The average solar wind speed naturally divides CMEs into fast and slow ones. Based on the relation between the acceleration and initial speed, we derive an empirical model to predict the arrival of CMEs at 1 AU.

Radio Signatures of Coronal Mass Ejection Interaction: Coronal Mass Ejection Cannibalism?

We report the first detection at long radio wavelengths of interaction between coronal mass ejections (CMEs) in the interplanetary medium. The radio signature is in the form of intense continuum-like radio emission following an interplanetary type II burst. At the time of the radio enhancement, coronagraphic images show a fast CME overtaking a slow CME. We interpret the radio enhancement as a consequence of shock strengthening when the shock ahead of the fast CME plows through the core of the preceding slow CME. The duration of the radio enhancement is consistent with the transit time of the CME-driven shock through the core of the slow CME. As a consequence of the interaction, the core of the slow CME changed its trajectory significantly. Based on the emission characteristics of the radio enhancement, we estimate the density of the core of the slow CME to be ~4 × 104 cm-3. The CME interaction has important implications for space weather prediction based on halo CMEs: some of the false alarms could be accounted for by CME interactions. The observed CME interaction could also explain some of the complex ejecta at 1 AU, which have unusual composition.

Near‐Sun and near‐Earth manifestations of solar eruptions

We compare the near-Sun and near-Earth manifestations of solar eruptions that occurred during November 1994 to June 1998. We compared white-light coronal mass ejections, metric type II radio bursts, and extreme ultraviolet wave transients (near the Sun) with interplanetary (IP) signatures such as decameter-hectometric type II bursts, kilometric type II bursts, IP ejecta, and IP shocks. We did a two-way correlation study to (1) look for counterparts of metric type II bursts that occurred close to the central meridian and (2) look for solar counterparts of IP shocks and IP ejecta. We used data from Wind and Solar and Heliospheric Observatory missions along with metric radio burst data from ground-based solar observatories. Analysis shows that (1) most (93%) of the metric type II bursts did not have IP signatures, (2) most (80%) of the IP events (IP ejecta and shocks) did not have metric counterparts, and (3) a significant fraction (26%) of IP shocks were detected in situ without drivers. In all these cases the drivers (the coronal mass ejections) were ejected transverse to the Sun-Earth line, suggesting that the shocks have a much larger extent than the drivers. Shocks originating from both limbs of the Sun arrived at Earth, contradicting earlier claims that shocks from the west limb do not reach Earth. These shocks also had good type II radio burst association. We provide an explanation for the observed relation between metric, decameter-hectometric, and kilometric type II bursts based on the fast mode magnetosonic speed profile in the solar atmosphere.

Tracing the topology of the October 18–20, 1995, magnetic cloud with ∼0.1–10² keV electrons

Five solar impulsive ∼1–10² keV electron events were detected while the WIND spacecraft was inside the magnetic cloud observed upstream of the Earth on October 18–20, 1995. The solar type III radio bursts produced by these electrons can be directly traced from ∼1 AU back to X-ray flares in solar active region AR 7912, implying that at least one leg of the cloud was magnetically connected to that region. Analysis of the electron arrival times shows that the lengths of magnetic field lines in that leg vary from ∼3 AU near the cloud exterior to ∼1.2 AU near the cloud center, consistent with a model force-free helical flux rope. Although the cloud magnetic field exhibits the smooth, continuous rotation signature of a helical flux rope, the ∼0.1-1 keV heat flux electrons and ∼1–10² keV energetic electrons show numerous simultaneous abrupt changes from bidirectional streaming to unidirectional streaming to complete flux dropouts. We interpret these as evidence for patchy disconnection of one end or both ends of cloud magnetic field lines from the Sun.

The source region of an interplanetary type II radio burst

We present the first observation of the source region of an interplanetary type II radio burst, using instruments on the Wind spacecraft. Type II radio emission tracks the motion of a CME-driven interplanetary (IP) shock which encounters the spacecraft. Upstream of the IP shock backstreaming electrons are observed, first antiparallel to the interplanetary magnetic field (IMF), and then later parallel as well. Langmuir waves are observed concomitant with the shock-accelerated electrons. The electron energy spectrum and Langmuir wave amplitudes are very similar to those observed in the terrestrial electron foreshock. From the connection times to the shock, we infer the existence and characteristic size of large scale structure on the shock front. The type II radio emission seems to be generated in a small bay upstream of the shock, and this may account for some splitting structure observed in the frequency spectrum of many type II bursts.

Characteristics of coronal mass ejections associated with long-wavelength type II radio bursts

We investigated the characteristics of coronal mass ejections (CMEs) associated with long-wavelength type II radio bursts in the near-Sun interplanetary medium. Type II radio bursts in the decameter-hectometric (DH) wavelengths indicate powerful MHD shocks leaving the inner solar corona and entering the interplanetary medium. Almost all of these bursts are associated with wider and faster than average CMEs. A large fraction of these radio-rich CMEs were found to decelerate in the coronagraph field of view, in contrast to the prevailing view that most CMEs display either constant acceleration or constant speed. We found a similar deceleration for the fast CMEs (speed > 900 km s−1) in general. We suggest that the coronal drag could be responsible for the deceleration, based on the result that the deceleration has a quadratic dependence on the CME speed. About 60% of the fast CMEs were not associated with DH type II bursts, suggesting that some additional condition needs to be satisfied to be radio-rich. The average width (66°) of the radio-poor, fast CMEs is much smaller than that (102°) of the radio-rich CMEs, suggesting that the CME width plays an important role. The special characteristics of the radio-rich CMEs suggest that the detection of DH radio bursts may provide a useful tool in identifying the population of geoeffective CMEs.

Origin of coronal and interplanetary shocks: A new look with Wind spacecraft data

We have investigated type II radio bursts in the solar corona using data from ground-based radio telescopes (>18 MHz) and from the Radio and Plasma Wave experiment (WAVES) on board the WIND spacecraft (<14 MHz). The wavelength range of the WAVES experiment includes the 2- to 14-MHz band, previously unobserved from space. We found that all 34 coronal type II bursts observed over a period of 18 months (November 1, 1994, to April 30, 1996), decayed within a few solar radii and did not propagate into the interplanetary medium. On the other hand, most of the accompanying type III radio bursts observed by the ground-based instruments were observed to continue into the interplanetary medium as the electron beams propagated freely along open magnetic field lines. Over the same period of time, other instruments on board the WIND spacecraft detected about 18 interplanetary shock candidates, which seem to be unrelated to the coronal type II bursts. This result confirms the idea that the coronal and interplanetary shocks are two different populations and are of independent origin. We reexamine the data and conclusions of Gosling et al. [1976], Munro et al. [1979], and Sheeley et al. [1984] and find that their data are consistent with our result that the coronal type II bursts are due to flares. We also briefly discuss the implications of our result to the modeling studies of interplanetary shocks based on input from coronal type II radio bursts.

Dust Detection by the Wave Instrument on STEREO: Nanoparticles Picked up by the Solar Wind?

The STEREO wave instrument (S/WAVES) has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.

Relation Between Type II Bursts and CMEs Inferred from STEREO Observations

The inner coronagraph (COR1) of the Solar Terrestrial Relations Observatory (STEREO) mission has made it possible to observe CMEs in the spatial domain overlapping with that of the metric type II radio bursts. The type II bursts were associated with generally weak flares (mostly B and C class soft X-ray flares), but the CMEs were quite energetic. Using CME data for a set of type II bursts during the declining phase of solar cycle 23, we determine the CME height when the type II bursts start, thus giving an estimate of the heliocentric distance at which CME-driven shocks form. This distance has been determined to be ∼1.5Rs (solar radii), which coincides with the distance at which the Alfvén speed profile has a minimum value. We also use type II radio observations from STEREO/WAVES and Wind/WAVES observations to show that CMEs with moderate speed drive either weak shocks or no shock at all when they attain a height where the Alfvén speed peaks (∼3Rs – 4Rs). Thus the shocks seem to be most efficient in accelerating electrons in the heliocentric distance range of 1.5Rs to 4Rs. By combining the radial variation of the CME speed in the inner corona (CME speed increase) and interplanetary medium (speed decrease) we were able to correctly account for the deviations from the universal drift-rate spectrum of type II bursts, thus confirming the close physical connection between type II bursts and CMEs. The average height (∼1.5Rs) of STEREO CMEs at the time of type II bursts is smaller than that (2.2Rs) obtained for SOHO (Solar and Heliospheric Observatory) CMEs. We suggest that this may indicate, at least partly, the density reduction in the corona between the maximum and declining phases, so a given plasma level occurs closer to the Sun in the latter phase. In two cases, there was a diffuse shock-like feature ahead of the main body of the CME, indicating a standoff distance of 1Rs – 2Rs by the time the CME left the LASCO field of view.