Argentina's heavy air raids today came after a lull that provided vital time for Britain to reinforce and consolidate its positions on East Falkland Island, and deploy land-based antiaircraft missiles that were used today for the first time in combat.

Since their initial invasion Friday, the British ground forces have been strengthened by emplacement of Rapier antiaircraft missiles, a "fire-and-forget" weapon that operates automatically, has its own guidance system and a range of up to 10 miles.

A British reporter at the beachhead said in a pooled report that he saw a Skyhawk shot down by a Rapier battery. He also said a turboprop Pucara counterinsurgency plane, flying from an airstrip on the island, was shot down by a four-man ground patrol using small arms.

Despite today's attack on the task force ships, analysts here expect the war gradually to shift from an air-sea confrontation to an air-ground one. The Defense Ministry announced that at least six of Argentina's fighter-bombers were downed today, bringing the total Argentine losses to more than 40 planes in combat and on the ground. It is believed that Argentine pilots are likely to have increasing difficulty now that Britain has consolidated and reinforced its beachhead.

Thus far, Argentine tactics have centered on its Air Force winning a war of attrition and in imposing heavy casualties to wear down the British will to fight on the home front.

Argentina's numerical air superiority, originally calculated at about 8 to 1, has been considered vital for it to hold the islands seized from Britain 51 days ago. But heavy Argentine losses, and the arrival of at least 20 Harrier jump jet reinforcements that brought the number of British fighter planes in the war zone to about 40, probably have brought Argentina's advantage down to about 2 to 1.

In addition, British military analysts maintain that the Harriers have superior maneuverability over the Argentine fighter-bombers, which are mainly 15- to 20-year-old models.

If British reports are accurate, Argentina has lost at least 20 Mirages and Skyhawks in the last three days. It is unknown if others failed to make it back to the mainland and had to ditch at sea because of damage or lack of fuel. Britain has acknowledged the loss of four Harriers.

On the basis that only 75 percent of the planes available are operational at any time, former air marshal Stewart Menaul estimated that Argentina may only be capable of putting about 60 planes in the air now as opposed to 30 for Britain.

Once airborne, they must face the possibility of opposition on land from the Rapiers and the shoulder-held Blowpipe missiles as well as conventional small-arms fire. It is also believed that Royal Air Force Harriers now can operate from airstrips of steel tracking put down by engineers.

In addition, the Argentine pilots still face sea attacks from Harriers based on the two aircraft carriers and a variety of sea-based missiles on escort ships.

Some analysts think Argentina soon may have trouble fielding enough trained pilots and may have difficulty in maintaining the jets and turning them around quickly for new sorties.

One analyst, who declined to be named, said that the loss of pilots is far more serious than the loss of aircraft. The pilot casualty rate may cause Argentina to reconsider its air strategy, he added.

One analyst refused to get involved in the numbers game, but said simply, "The Argentines have had a hell of a reduction in their capability. With the new land defenses the pilots face prodigious odds. It will be very much more difficult for the Argentine Air Force."

Morale of the pilots might be suffering, he said. They have to fly practically the maximum combat radius over the ocean on a single engine, face greatly improved antiaircraft defenses and know there is no casualty evacuation if they have to bail out into the freezing sea. "If they ditch, they've had it," he added.

The Rapier is one of the new generation of air defense missiles similar to the Sea Wolf on some of the British frigates. Both fire automatically and home in on their target with their guidance system unless they are restrained by their crew.

The Rapier, however, is an "area defense system" as opposed to the simple "point" defense offered by the Sea Wolf, which scored hits on Argentine planes earlier this month. The Sea Wolf can only protect the ship it is based on or one within 1,000 yards.

Unlike the Sea Wolf, the Rapier is capable of distinguishing between friend and foe, but Royal Air Force pilots are known to be uneasy about flying near the missile batteries anyway.

"The Rapiers provide an umbrella of cover" that cannot be obtained from a sea-based missile system, the analyst said.

The Argentine pilots are also liable to face more Sea Harriers since the placement of RAF Harriers on land for ground attacks frees the jump jets on the carriers for air defense exclusively, if necessary.

In other action today, the Defense Ministry said British aircraft attacked the settlement and airstrip at Goose Green, about 40 miles south of the British beachhead. A commercial vessel used by the Argentines to ferry troops, ammunition and supplies around the island was also run aground and abandoned by its crew after being threatened by a task force ship.

Alistair McQueen, a correspondent for the London Daily Mirror, filed a pool dispatch from the San Carlos beachhead describing the scene as the Argentine planes attacked today. He said he had to dive for cover three times as the jets swept in over the island.

"The commando unit to which I am attached has been sending out fighting patrols every night, pushing further forward in its attempt to get into contact with the enemy. But they appeared to have disappeared," McQueen wrote.

"There is no evidence of any Argentinian troops massing for an assault against us, as claimed by the junta in Buenos Aires.

"The only Argentinians we have heard about so far were two more who walked into parachute regiment positions flying white flags at dawn today. The men were hungry, wet and frightened and without their weapons."

A survivor from the sunken frigate HMS Ardent aboard the Canberra told how waves of Mirages attacked the ship Friday.

"When the last wave of five dived, we had only a team of machine-gunners led" by the stores manager, a civilian who learned to fire in the Army, he said.

Television programs also showed Argentine film reportedly taken last Thursday at Stanley, an indication that some aircraft are still breaking the British blockade. The film showed defensive positions, artillery, antiaircraft batteries, gunboats offshore and troops carrying rocket-propelled grenades.