In an effortto understand howactive regionsareheated,SOHO transition-region data arebeing explored to studyvarious

aspectsof variability of active region emission linesand structures.For example,CDSobservationswith 30 scadence show

oscillations of He I (2x10⁵K) and O V (2.5x10⁵K) intensities taking place on time scales of 5-10 min.Amplitudes of the order

of 10-15% are common,but intensity burstswith a 50% amplitude taking place in the legsof a magnetic loop have also been

recorded.CDShasalso observed othertransientphenomenathatarepossible signaturesofheating taking placein alarge

coronal loop over one hour period, resulting in changes in the intensity distributions along the loop; an appearance of a loop seen

in all temperatures from 9x10⁴to 10⁶K, which was seen only for 36 minutes and decayed afterwards; and explosive brightenings

taking place in small loops in an active region.

Ratios of EIT images can yield temperature information in the range 1.0 - 2.0 MK. Full-disk temperature maps(Figure 6) have

revealed a startling result: the area of the corona in which active regions are responsible for enhanced heating is much larger than

the bright loop systems visible in a single waveband ( i.e., where the density is enhanced). Work is in progress to obtain a better

temperature calibration for EIT ratiosin order to determine the netincreasein coronalheating in the lower-density regions.

Jetsand NetworkPhenomena.Jetsandotherfeaturesofthequietsolarnetworkmayprovidekeystothenatureofboth

atmospheric heating and transition region mass balance.Both SUMER and CDS have studied jets and explosive events exten-

sively.The SUMER observations have for the first time provided reliable statistics on the temperature and velocity distributions

oftheseeventsaswellastheirbirthrates.SUMERobservationssuggestthatsomejetsdisplayboththemorphologyand

physical characteristics of magnetic reconnection.CDS has detected jets or explosive events at coronal temperatures.These are

pocketsof flows,with speedsover a fewtensof km s^{- 1}to hundredsof kms^{- 1}.However,they are usually seen in active region

loops and are notas common asone may have expected fromHRTS or SUMER data from cooler lines.

CDShas discovered so-called "blinker" events.Theseare sitesin the network which show significantbrightening over a few

minutes at intermediate temperatures.They are found anywhere over the quiet Sun--with roughly 3000 present on the Sun at

any time.Each containsonly about10²⁵ergsof thermalenergy butthey maybe thevisible partof moreenergetic events.

Indeed, their global nature suggests that they may be related to global processes such asheating.

MHD Wave Heating.MHD wavesare one of the leading contenders for heating the solar corona.One way to search for these

is to examine in detail the profilesof coronal emission lines.Using this technique, UVCS has measured velocity distributions

of several particles of different mass and charge-to-mass ratio in the extended solar corona.At the base of equatorial streamers,

which have the highest densities in the extended corona, the observed velocity distributions along the line of sight (LOS) nearly

correspond to a single kinetic temperature.Large departuresfrom a thermal distribution occur in lower density regions of both

equatorial streamers and polar coronal holes.For example, at heliocentric heights of 3 R _sunin polar coronal holes, O ⁵⁺velocities

at1/e (v_{1/ e}) are morethan 2.5 timeslarger thanthose of protons(compared to theten timessmaller O ⁵⁺value expected in a

thermalized plasma).In such regions, the observed v _{1/ e}for O⁵⁺of 575 km s^{- 1} along the LOS would correspond to a temperature

of 3.2 10⁸K in a thermalized plasma.The v_{1/ e}of the protons would correspond to a temperature of about 3 10⁶K and the electron